Rebuilding a Fender Deluxe Reverb
Reissue Amplifier
Featuring MikeyMann Amplification
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(Updated on 2004-09-06. )
http://waynereno.com/index.htm
01 - Overview
My name is Wayne and this article is about my experience with MikeyMann Amplification turning my Fender Deluxe Reverb Reissue into a reborn, boutique amp. Casting aside the printed circuit board and stock transformers, Mikey breathed new life into this amp with thoughtful improvements to enhance the sound and serviceability.
Mikey’s rebuilt
amp (on left) and Wayne’s stock Deluxe Reverb Reissue (on right)
I’ve written the following account in chronological order so that you can share my sense of discovery of the intricacies of this amp rebuild and the commensurate enhancement of my tube amp electronics knowledge. Keep in mind that while I am a well-informed amateur with a fair amount of book learning, I’m not an amp tech. Since tube amps are a major hobby of mine, I’m always glad to gain new tube amp knowledge and this experience has been a true pleasure in this regard. I hope you enjoy the journey through the rebuilding of this tube amplifier as much as I enjoyed writing it. From one tube ampoholic to another . . . . .
To aid in the navigation of this article, here is a table of contents, which also acts as a link to the various sections of the article. Click on a link to jump to that topic, or read on for the full story.
02 - Introduction
Well, I must admit to an almost morbid fascination with the Deluxe Reverb Reissue (DRRI) amps. I’ve owned three of them over the years. I say “morbid” because each of them has died (primarily related to blown tubes) at some critical point while I’ve been performing. Each time, I’ve sworn them off but like a phoenix rising from the ashes, I’ve come back because the sound of a Deluxe Reverb with its 6V6 power tubes and wonderful reverb hold a special appeal to me. I have found the Fender DRRI as it comes straight from the factory sounds pretty darn good for my playing style and even better with an upgraded speaker, but as with many things, there is plenty of room for improvement.
RCA 6V6
The first time I acquired a DRRI, I purchased it brand new at a yearly special
sale at a local music store. I played
it as my main amp for about 10 months until at a gig, one of the Sovtek 6V6 tubes
failed and took out one of the 470-ohm/1-watt screen resistors.
So I took it to my amp tech, Rick, for servicing. It was my first look at the interior and under Rick’s instruction, I learned about the quality of the printed circuit board, the sometimes problematic wiring traces and the difficulty servicing it. As some of you may recall, at a point chronologically after my first DRRI amp, Rick performed a wonderful restoration of my 1971 Fender Princeton Reverb. You can read that tale of transformation on the alt.guitar.amps home page at:
Rebuilding a 1971 Fender Princeton Reverb
Rick patched up my DRRI and I played it for a little while but decided to start looking for better-built amps with a Deluxe Reverb vibe. I sold the DRRI to buy a Tone King Continental head and cab, an amp I still own today.
An opportunity to buy a used DRRI at a good price came along so I bought it. I upgraded to 6V6EH tubes and used the standard speaker. It too gave out at a gig due to a failed tube. I had it fixed but sold it within a couple of months, swearing them off as too unreliable.
Then Tom, a fellow I’d been playing music with for a number of years, left the area and asked if I wanted to buy his DRRI. It was in great shape and was too good a deal to pass up so I bought it and installed a Reverend All-Tone 12” speaker. Guess what, I was playing it about 9 months ago and had the same problem; at a high volume, one of the 6V6EHs failed and took out a screen resistor. My amp tech fixed it up by installing 1K/5W screen resistors and re-soldered some loose grounds and other components on the PCB board.
Vowing to do something about the amp, I got in contact with Mikey (Mike McGuirk) of MikeyMann Amplification. I’d been aware of Mikey’s work on DRRIs through his frequent posting on the newsgroup alt.guitar.amps. He’d done some nice work on his DRRI, gutting his amp and stuffing it with a Hoffman turret “PTP” (Point-To-Point) board and upgraded components. I’d exchanged quite a few bulletin board posts with him on the topic and liked what I’d read. I’d seen examples of his work on the Amplifier Chassis Pics website at:
Of course many of you that are reading this would think I’d have been better off initially buying a Blackface or Silverface Deluxe Reverb. You’re probably right, considering I’m on my third DRRI. But when I first bought the DRRIs, I wasn’t as knowledgeable about tube amps as I am today and with just a little over $300 in the current DRRI, I’ve decided to try something new.
A recent sampling of sold eBay transactions revealed that original Blackface (pre 1968) Deluxe Reverbs had an average selling price of $1255. The downside of purchasing these amps is they often need $250 - $500 in restoration work and tubes and you are still left with an amp that has many 40-year-old components.
Read on and I think you’ll find that buying a used DRRI and getting a MikeyMann Amplification rebuild is a cost effective way to get the Deluxe Reverb sound we love using modern, reliable components that sound great and still have all the panache of a boutique amp. I wish I had done this the first time I bought a DRRI :>)
03
- Rebuild Discussions
Beginning in February, I’ve had long conversations with Mikey about rebuilding my Deluxe Reverb Reissue. We have exchanged lengthy emails with ideas about the rebuild and the conversations have helped clarify my understanding of his concepts as well as his understanding of what I like to hear in an amp. These conversations have also helped educate me about component choices and what they will do to the sound.
As it turns out, Mikey and I have very similar tastes and both like the basic Deluxe Reverb sound. When you talk to Mikey, you’ll soon find out that tweaking an amp to an individual player’s style, their guitar choices and other equipment selections is the key to getting the best sound for the player.
The circuit he uses as the starting point for his modifications is (not surprisingly) the one used in Deluxe Reverbs, which is called the AB763. It is used in a variety of Blackface Fender amps including the Deluxe Reverb, Super Reverb, Vibroverb, Virbrolux, Twin Reverb, etc. This means if you have a Blackface Fender reissue amp, chances are the AB763 circuit is used. Of course, there are variations to the circuit depending on the number and type of power tubes in a particular reissue amp.
Often, when I want to find detailed Fender amp information,
I go to the Fender Amp Field Guide. There's a host of great information
about all of the vintage Fender amps, plus, you can find copies of the chassis
layouts and schematics, which is where the AB763 diagrams on this page came
from. Check it out.
Deluxe Reverb
AB763 chassis layout (click picture for full size, external link)
Deluxe Reverb AB763 schematic (click picture for full size, external link)
Although Mikey has previously made a variety of
different circuit boards, he decided to specialize in rebuilding the Fender
reissues, specifically the Blackface and Tweed Bassman amps, and the reissue
stand-alone reverb unit. Mikey says he may offer other circuits in the
future and is considering additional performance upgrades for popular Fender
amps not currently in the reissue line such as the Blues Jr., Pro Jr., Blues
Deluxe, Blues DeVille, Hot Rod Deluxe, Hot Rod DeVille, etc.
He got his start building amplifiers by using Hoffman Board Kits,
which are no longer manufactured by Doug.
Mikey has been a long-time fan of Doug Hoffman’s circuit designs and
continues to get parts from the Hoffman Amplifiers website. In fact, some Hoffman parts have found their
way into this project. You can check
out the parts for sale and access other excellent tube amp information at:
In one of many emails I received from Mikey, he offered these comments regarding the people that inspired and informed him during his quest to build tube amps:
"I have to say, that I have a lot of respect for Doug.
I learned an incredible amount about amp building from his website, which has a
ton of information that he offers FREE of charge. He absolutely has one
of, if not the best, amp building forums around. There aren't too many
experts out there that offer their knowledge to others free of charge.
Doug Hoffman and Randall Aiken are two that come to the top of my
mind. I've learned a lot from both of them and I hope to be able to
"pass the torch" by offering the knowledge that I've gained, and hope
to gain, with others as well."
I can certainly relate to Mikey's
comments about Randall Aiken. I purchased one of the first Aiken Invader
amps in the early summer of 2001 and have been gigging with it ever
since. It’s a wonderful amp.
Randall's website is chock-full of technical information. I
recommend checking it out at:
During discussions about
rebuilding my amp, I had the opportunity to consider and
select some optional mods to Mikey’s “stock” rebuild. For example, I chose the option for a Triode/Pentode
switch for lower volume situations.
I learned that Mikey likes to modify the negative feedback loop from the standard AB763 circuit because he has found a sweet spot for this amp to increase gain and harmonics.
He suggested modifying the normal channel for more midrange, overdrive and a more Marshallesque tone. On his personal amp, he added a pot in place of the extension speaker jack to allow full-range control of the midrange. While I still want to keep the extension speaker jack, the plan is to test different midrange settings on his amp with my guitars to find the optimum setting, which will be set at a fixed value by placement of a properly-speced resistor in the tone circuit of the Normal channel.
Internally, in addition to a new circuit and filter cap boards, Mikey said I could select the option to add bias points on the back of the chassis with adjustments made via the bias pot accessed through the bottom of the chassis. Normally, the bias pot is located on the main circuit board. The bias can still be adjusted without the bias test points but the chassis has to be removed to get the measurements, or you can use a bias probe (purchased separately) in the power tube sockets. Although it costs more, I chose the option with the external bias test points, which will allow me to change output tubes without taking the amp to a tech.
There will be 1K/5W screen resistors on the 6V6 tubes to prevent the kind of failures I experienced on all three of my DRRIs.
Mikey suggested a phase inverter (PI) mod to limit the farting out of the speaker at high volumes. Lowering the PI’s cap value does this. Mikey has refined the sound of his circuits by making subtle changes here and there rather than a couple of major changes. For example, to balance the lower cap value in the PI, which raises the low frequency limit but still keeps it below what the speaker can reproduce, Mikey increases the PI coupling cap to lower the frequencies going to the PI to compensate by allowing a bit more midrange / fullness through.
He suggested a mod to run the Normal channel through the tremolo and reverb circuits, putting both channels in phase. This will let me use an A/B/Y pedal to run the channels separately; with the Normal channel having a bit more gain and midrange, and the Vibrato channel retaining the cleaner Deluxe Reverb sound. With both channels in phase, I will also be able to run them together for even more sonic goodness.
Mikey’s circuit uses the pulsating bias tremolo circuit of the 63' Vibroverb and not the optoisolator type that is used on most Blackface Fenders. This circuit alters the bias of the power tubes instead of an on/off optoisolator.
He’s also going to adjust the reverb circuit by using a 100K audio pot in place of the 100K linear pot to give more fine control so the reverb won’t jump too quickly from subtle to surf.
There are other options to consider when getting a rebuild done by Mikey. He is a BIG fan of Mercury Magnetics iron. These high-quality parts apparently have a great effect on the tone of the amp and Mikey insists on using Mercury Magnetics output transformers and chokes in his rebuilds. Mercury iron can also be used for the reverb transformer and power transformer. I initially chose the option to add the reverb transformer, but as you’ll see in a later section, I’m convinced the Mercury power transformer is a worthwhile upgrade.
I certainly learned a lot in my email and phone conversations with Mikey. I decided I wanted to document this rebuild so in addition to saving all the emails, I asked him for digital photos of each step of the building process. He agreed and what started as my personal record keeping has ended up as this full-blown website.
When all is said and done, I’ll have boutique amp quality for quite a bit less than the cost of a new boutique amp and this one will be personally tweaked by Mikey to my sonic preferences. Plus, it'll be like a wolf in sheep's clothing, housed inside the Fender chassis and cabinet. Sounds good to me!
04 - Visiting MikeyMann Amplification
In July, I traveled to Baltimore, Maryland (USA) with my DRRI to meet with Mikey. I arrived in the early afternoon and took copious notes to share the experience with you. Here’s how we spent the day, including a lot more information that I learned about rebuilding this DRRI amplifier.
Once I arrived, I carried my two favorite Hamer guitars and my DRRI amp in from the car and met Mikey for the first time, and his two cats. One cat is unabashedly friendly and the other one is quite reserved and timid. After getting the animal familiarities out of the way, Mikey and I took some time to get to know each other.
Mike McGuirk (aka Mikey) in his shop
I asked him about his business name, MikeyMann
Amplification. He shared with me that
once when visiting his oldest niece,
she spontaneously said, " Hey, Uncle Mikey Man!" Then, whenever
she saw him, she’d call him “Uncle Mikey Man”. This term of endearment
has been used by all of his nieces over the years, which is how the name
MikeyMann Amplification came to be.
We then talked about our love of tube amplifiers, guitars and the musicians that have influenced us. We shared war stories of bands we had played in over the years and most importantly, discussed the amps we’ve played in the past and the amps we own today. We also discussed the state-of-the-art in modern tube amp construction and spoke of the knowledge we’ve both gained by participating in the AGA newsgroup (alt.guitar.amps).
We also discussed a host of Internet sites, tube amp books and interactions with other musicians that have helped us out. I’ve learned a lot from The Gear Page website at:
Mikey highly recommends The ToneQuest Report for detailed
reviews on amps, guitars and other musicians’ toys. He says,
“It's my favorite publication, even above Guitar
Player. I look
forward to receiving it each month, and I devour it for its information and
knowledge. I save all of my issues so
that I can refer back to specific articles and information. It's also cool to read what some of the
great players are using, how they have their rigs set up, etc.”
I like it too and buy it off the newsstand at music stores. You can find out more about it here:
Mikey is a joy to talk to about these topics and we spent the first couple of hours in animated conversation. Mikey took the time to talk about his vision of rebuilding reissue amps with modern construction techniques using the best of modern components. His vision is to build amps for working musicians, pros, semi-pros and serious players who want the best possible tone and top-notch reliability. He makes a compelling case for using the cabinet, chassis and some of the components of the Fender reissue series instead of buying an old Blackface or
Silverface amp and only upgrading some of the components, leaving 30 – 40 year old components in place that could fail at an inopportune time.
I thought you might enjoy reading Mikey’s vision as stated in an email he wrote me:
“Lots of players want/need great tone, but
can't afford the thousands it usually takes to get a handmade or boutique amp,
or even certain vintage amps whose costs have skyrocketed over the last few
years. My goal is to offer great tone
and make it more affordable to a greater number of players. Also, many gigging musicians and pros don't
want to take their vintage gear on the road for fear that it will get stolen or
destroyed. By rebuilding reissue amps
like the Deluxe Reverb, they get the great vintage looks and tone but don't
have to worry about a priceless vintage amp getting stolen or damaged. If so, they can simply get another one!”
“My personal feeling is that if someone does buy
a 40 year old vintage amp, chances are it will have new electrolytics, probably
some new coupling and tone caps, new resistors (at least plate resistors),
maybe a new output transformer, new tubes, maybe a new speaker, maybe even a
new power transformer. So, does the
player "really" have a vintage amp? I say that they have a
vintage chassis, vintage cab but not a "vintage amp". Once you start replacing a bunch of stuff on
a 40-year-old amp, it ceases to be a "vintage amp", in my humble
opinion. You still have a vintage
chassis and cab, but with a bunch of new parts. So, you're paying all that money for a "vintage amp",
but you're not totally getting a vintage amp, just a vintage chassis with a
bunch of new parts. Where's the logic
in that?”
After our very pleasant and wide-ranging conversation, it was time to delve into the details of the amp rebuild. What follows is a section-by-section account of what is planned.
05 - Design Details
The stock DRRI uses printed circuit boards with smaller components and the pots and jacks are mounted to the green PCB. Mikey’s amp uses turret board construction and the pots and jacks are mounted to the chassis and hand wired. Look at the differences in the capacitors, resistors and wiring (instead of ribbon cables). A picture is worth a thousand words and this photo shows the dramatic differences between the two amplifiers.
Mikey’s rebuilt amp (top) and Wayne’s stock Deluxe Reverb Reissue (bottom)
Mikey and I went into the shop and started talking in detail about all of the planned changes to my amp. He took the time to patiently answer all of my questions as we went through each section. I really learned a lot during this session.
Starting with the main board, I learned that Mikey uses a 1/8” G10 board and turrets. Of course, I’ve seen turret board construction in other amps but I didn’t really understand how it is done. Mikey showed me how a large board is measured and cut to size. Then a circuit template (plastic) and a drilling jig (metal) is laid on the G10 board. The template indicates which holes need to be drilled and the jig is used to guide the drill bit as it cuts the board. Once all the holes are drilled, metal turrets are placed through the holes and a special tool is used on the drill press to flare the turrets in the board so they grip firmly. This is a time-consuming process but it makes for very solid construction.
Mikey talked about his existing circuit board template used on the amp shown in the above picture but he has since refined the circuit and construction techniques so he will be using a modified template for my amp. As we worked through the sections of the amp, he took the time to show me how the existing circuit in his personal rebuild would be modified for my build. Hey, I’m getting all the latest whiz-bang improvements!
One of the improvements is the use of 18-gauge buss wire,
wrapped in a Teflon shield for the power rail and ground rail on the
board. This will help insulate power supply noise from the signal
wires that are in close proximity.
Here’s what Mikey has to say about the use of Teflon shielding.
“Teflon is one of the best insulators available and
by using Teflon as a shield on the power and ground rails adds additional
protection for any hands that happen to be in the chassis! High voltage,
bare wire, and hands don't mix too well!”
Mikey will be using the optional Mercury Magnetics iron in
my amp for the output and reverb transformers and the choke. (Note:
A few days before picking up my amp, a decision was made to talk to Mercury
about removing the DRRI power transformer and replacing it with a Mercury
Magnetics power transformer. This makes
for a good story, which you can read in a later section.)
Mercury Magnetics is highly regarded and they’ve taken the time to reverse engineer the best sounding transformers. Mikey explained they use interleaved windings, which the Fender transformers in my stock DRRI don’t have. If nothing else, Mikey strongly recommends using the Mercury output transformer because it adds so much more tone for a few more dollars. You can find out more about this company by following this link:
Here’s a photo that shows the DRRI iron compared to the
Mercury Magnetics iron.
Stock DRRI chassis
(top) and replacement Mercury iron (bottom)
The choke and the output transformers from Mercury Magnetics are significantly larger than the stock iron. I wasn’t clear on the purpose of the choke so Mikey explained that it is an inductor, which means it opposes changes in current. As current decreases, it lets more through and vice versa. The inductor smoothes the current whereas a resistor doesn’t offer any flexibility in how much current it lets through.
On the back of the DRRI, there is an unused switch that was used to change the polarity of the AC coming into the amp on the old Blackface Deluxe Reverbs. Since all modern amps use a grounded AC connection, this switch isn’t needed. Fender put this switch in just to make the back of the amp look like an original.
On my amp, this SPST (single pole, single throw) switch is replaced with a DPDT (double pole, double throw) switch for the Triode/Pentode mod. This changes how the 6V6 tubes operate. If someone chooses a different mod like turning the negative feedback loop of and on, the SPST switch can be reused.
6V6s are pentode beam power tubes. These do the “heavy-lifting” power amplification in the amp. There are five main parts inside a pentode tube, which are the cathode, grid, screen grid, suppressor grid and plate. The heater in the tube is powered by filament wiring coming from the power supply. These heat up the cathode, which starts shedding electrons causing DC current to flow through the tube. The very low signal from the guitar goes into the preamp section of the amp and the preamp tubes do the initial signal amplification and tone shaping. This signal comes out of the preamp section and is introduced into the power tubes at the grid. Here’s a scan from the RCA Receiving Tube Manual.
Pentode circuit showing the parts of the tube (inside the circle) and the signal path
The Deluxe Reverb uses a method called “fixed bias” by making the grid negative with respect to the cathode when no signal is present on the grid. Depending on the amount of negative DC voltage on the grid, the tube will let more or less current pass on through. The bias pot on the bottom of the Deluxe Reverb chassis is what is used to make this adjustment.
Since the bias voltage is negative, more negative bias voltage to the grid restricts the electrons coming off the cathode, which restricts the current flowing to the plate. Less negative bias voltage to the grid allows more electrons to pass to the plate. With less current going to the plate, the amp is considered to be biased “cold” and with more current it is biased “hot”.
The screen grid helps keep the electron flow stable by reducing oscillations. The suppressor grid deflects secondary emissions back to the plate, which results in more power. The plate receives the electrons coming from the cathode. As current flows from the cathode to the plate, the varying preamp signal on the grid is amplified big-time and then it flows into the output transformer, which drives the speaker.
Mikey explained that when the Triode/Pentode switch is thrown, the screen grid is tied directly to the plates of the tubes, essentially eliminating the suppressor grid. This reduces the power of the amp and changes its tone and feel. The volume is lower, the tone is smoother and darker and the breakup happens sooner. It’s a nice thing to have for lower volume situations, for bright guitars and probably for players who like jazz. The nice thing about this mod is that when the switch is set for Pentode, the response is exactly the same as a Blackface Deluxe Reverb.
06 - More Details
In the last section I talked about 6V6 tubes for power amplification. I thought you’d enjoy seeing this photo of two 6V6 tubes; the type of power tube used in Deluxe Reverb amps.
A matched pair of NOS, NIB Visseaux 6V6 power tubes from Lord Valve
These particular 6V6’s are some of
the best ones ever manufactured. They
were made in 1951 for the French military.
NOS means they were sold as New Old Stock (never used). They are also NIB, which means they are
essentially New In the Box. These are
beautiful sounding tubes that compare favorably with the famous RCA blackplate
6V6’s. For new tube manufacturers of
6V6 tubes, I like Electro-Harmonix and JJ Electronic. I usually purchase
my tubes from Lord Valve of NBS Electronics in Denver, Colorado
(USA). You can get on his detailed tube mailing list for the latest
information and pricing:
One of the things I don’t like having to do is to take my amp to a tech for biasing when changing power tubes. The DRRI has a bias adjustment on the bottom of the chassis, which is a good quality CTS 10K bias pot so Mikey reuses this part in the rebuild. As I mentioned, Mikey is adding an optional mod to my amp so that I can bias it myself without opening up the amp and exposing myself to lethal charges or having to buy an external bias probe. Mikey solves this by adding three test points on the back of the chassis so I can set my own bias with a voltmeter using the cathode resistor method.
The way this works is a 1-ohm 1% resistor is placed between ground and pin 8 on the power tubes. When you plug a voltmeter into the test jacks, it’ll measure the voltage drop across the resistor. When I pick up my amp from Mikey, we’ll play with the bias on the tubes to find the spot that sounds best for me and is within the operating parameters of the 6V6 tubes. When it is time to replace my power tubes, I’ll buy a matched pair and then re-bias the amp to these same settings. This is really a nice way to make the amp user-friendly.
Mikey and I had been knee-deep-in-the-hoopla for 3.5 hours and we decided it was time for a break. We went to the kitchen and got a couple of Cokes, ice and glasses and sat outside to relax a bit. I continued to ask Mikey about the technical details of the rebuild and about tube amp electronics.
Mikey told of his time in the military and his assignment in Alaska where he played concerts with his band. At one big concert, he played to a huge crowd sharing the bill with Little Feat, Tower of Power, W.P. Brennan from Australia and 14 other local bands. It was like a mini Woodstock and it sure sounds like it was a lot of fun. After this brief respite, we returned to the shop and picked up where we’d left off.
Next up was a discussion of the typical negative feedback loop (NFB) used in the Deluxe Reverb and the tweaking Mikey has done to get the best response out of it. I learned that a global NFB loop is created by taking the signal from the output transformer at the positive terminal of the speaker jack and sending it back through a resistor to the phase inverter, which is at point earlier in the circuit before the power tubes. It is called negative feedback because this output signal is 180 degrees out of phase with the input signal.
Mikey experimented for a long time to find just the right resistor so that it wouldn’t mess with the clean sounds except making them a bit fatter with more harmonics. It also helps the tone when the amp is pushed hard, giving the amp more gain and girth. I was to experience this later when we started trying out the amps. Here’s what Mikey had to say about his modification to the NFB loop.
“This acts like a fixed Presence/Resonance control. Presence usually only allows the higher frequencies to pass through without going through the NFB circuit, and Resonance usually lets the lower frequencies through without going through NFB. The resistor mod that I'm using works with all of the frequencies, it doesn't only pass either high or low frequencies. It restricts the amount of signal that goes through the NFB circuit so that it leaves a portion of all of the frequencies to be reproduced naturally, without NFB. NFB reduces distortion and makes for a cleaner tone. Since a portion of the tones aren't going through the NFB circuit, they're reproduced fully, with more harmonics and Phatness!”
Regarding the phase inverter (PI), Mikey does some mods to this circuit too. This includes putting in a very high-quality coupling cap leading to the PI because all of the signal from the entire preamp section, reverb and tremolo pass through it on the way to the phase inverter. The PI caps themselves are also of high-quality. Here’s what Mikey has to say about his changes to the phase inverter circuit.
“The PI caps were modified to prevent the "farting" when playing at high volume. The PI coupling cap was actually raised a bit, to compensate a little for the PI cap change, and to let some more midrange/fullness through from the preamp circuit to the PI. My philosophy is that it's better to make several smaller changes/mods throughout the entire circuit than to just make one large/huge change and throw everything else off. Where I make one change, I usually try to make another small compensating change somewhere else in the circuit.”
Another enhancement Mikey makes from the stock DRRI is that he removes the Fender pots and jacks attached to the PCB and replaces them with new pots and jacks; wiring them directly to the appropriate parts of the circuit. He uses CTS and Alpha pots and Switchcraft jacks.
CTS pot (left),
Switchcraft jacks, Alpha pot (right)
The Alpha pots fit the existing holes but in order to use CTS pots, the holes must be enlarged. To do this, Mikey uses UniBits (step drill bits). His design calls for using shoulder washers to insulate the input jacks from the chassis to help keep the amp quiet. The input jack holes have to be enlarged to 1/2“ to accommodate the insulating shoulder washers.
Mikey puts the Normal channel in phase with the Vibrato channel. If this isn’t done and you try to play the amp when using both channels simultaneously, the sound is really puny since they are out of phase. This mod also has the advantage of routing the Normal channel through the tremolo and reverb circuits, which isn’t normally available on a Deluxe Reverb. The Normal channel is also modified to give Marshallesque tone and gain. He explained his goal is to add an extra midrange emphasis and allow the circuit to overdrive sooner.
During our conversation, I became curious about the use of different types of capacitors and how they are used to modify tone. Mikey explained to me how a capacitor works and showed me examples of “film and foil” caps and “metallized” caps. He had a couple of them disassembled and he used them to help me understand how caps are constructed. I now know that film and foil caps use two strips of film and two strips of foil, with a set of film and foil strips creating one “layer”. Metallized caps have two strips, each with an aluminum conductor atomized and vapor deposited on dielectric film. In both cases, the two layers are rolled up together so there is an anode conductor and a cathode conductor separated by a layer of film.
Two different methods of construction are evident in these two types of capacitors
Film and foil caps are much bigger than metallized caps. You can see this in the picture above. Both caps have the same .03uF/600V rating, but the polypropylene and foil cap is much larger/rounder than the metallized polypropylene cap on the right. On the left, the metal foil is less shiny and crinkled looking when unraveled. On the right, the metal looks very shiny and smooth since the metal was sprayed directly on to the dielectric (the polypropylene film).
The way a capacitor works is when power is applied from the positive and negative wires from the power supply, it becomes charged (one of the layers becomes positive and the other negative). Even with the power supply disconnected, the capacitor will continue to hold a charge. How much of a charge it can hold is measured in Farads (F). Another measure is the voltage rating, which tells you how much DC current the capacitor will block. In the above photo, the capacitors are rated at .03 microfarads and 600 volts.
While capacitors will block DC current, they will allow AC current to “flow” through. In actuality, capacitors cycle through charges and discharges somewhat like a very fast, rechargeable battery, which appears to be a flow of AC current. Interestingly, this cycling happens to let high frequencies pass through more easily than low frequencies. This means that capacitors can be used to shape tone in addition to passing an AC signal from one circuit to another without letting any DC current to get through.
There are many types of materials used for cap construction but polypropylene and polyester (foil or metallized) are the most common. Other examples of capacitor construction are paper/oil, polystyrene, polycarbonate, mica and ceramic. Mikey had this to offer on his choice of capacitors (note the preparation he did for my visit based on our voice and email correspondence).
“I
generally prefer polyester caps overall as they give off a "warmer"
tone/more midrange, which is especially nice in a bright Fender circuit. But I changed the caps in my amp for your
demo and used polypropylene Orange Drops for the PI caps and PI coupling
cap. I also used them in the normal
channel tone stack and a couple of other places since you like a brighter tone. Polypropylene gives off more highs and more
lows to my ears. Those were
polypropylene/foil caps, 716P's.
Polyester, either metallized or foil, tends to give more mids and upper
mids.”
Mikey explained a technique he will to use for the building of my amp. He calls it “Cap Stacking”, which is the use of a combination of caps of different types wired together to get the tone and grit characteristics to best fit his redesigned circuit and my particular tone preferences. I really enjoyed this part of the discussion and learned a lot I didn’t know about caps. As you can see, there’s a lot of attention to detail that goes into the design of Mikey’s amps.
There is an option Mikey uses that I did not select. On his amp, Mikey added a pot on the back of the chassis to the Normal channel to have variable control of the midrange EQ. This lets you dial-in just the right amount of mids for your particular guitar and for the sound you are going for. Mikey installed this extra pot in place of the extension speaker jack. Since I sometimes like to use a Celestion Alnico Blue extension speaker, Mikey said that when we try out his amp, I can find the sweet spot of the mids on his amp that best suits my taste preference. He’ll measure the resistance of the pot at that setting and just build it into the circuit of the Normal channel, leaving the extension speaker jack available.
The tremolo circuit is different from the stock DRRI circuit. His tremolo circuit uses the bias to pulsate the power tubes to generate a smooth tremolo effect rather than the on/off type found in the DRRI, which uses an optoisolator. He also slows down the tremolo circuit and raises the intensity a bit to get the most pleasing sound.
The reverb circuit is also significantly different from stock in several ways. The DRRI uses lower quality, PCB housed RCA jacks, which Mikey replaces with heavy-duty RCA female jacks affixed directly to the chassis. To do this, another hole has to be drilled in the chassis but the quality is worth it. I chose the option to use a high-quality Mercury reverb transformer. As anyone that uses a Fender amp with reverb knows, if you turn the reverb knob up past 2, it gets pretty boingy and surf-like. This is because of the use of a linear pot on the stock DRRI. Mikey uses an audio pot for this application so the user can have finer control of the reverb by allowing more incremental changes. Here’s Mikey’s explanation to me of the differences between linear and audio pots.
“Linear
pots are the same price as audio pots.
It's in the way that the pots function that makes the difference. The resistance of a linear pot is, well,
linear! It's in equal increments. For
example, if you have a 100K linear pot, and you turn the knob to the halfway
point, the resistance will be 50K. An
audio pot functions differently, but it functions closer to the way that our
ears hear. For the same 100K pot, the
halfway point may only represent about 20% of the total resistance, or about
20K.”
“As the
knob rotates, the resistance goes up more, then finally arrives at the same 100K
total resistance value. But, it's that
first half of the audio pot that gives us finer control, to get those
"in-between" values, and again, human ears "hear" in audio
fashion, not linear. That's one of the
reasons that on the Fender reverbs that use linear pots, it sounds like it's
"all on" even when the knob is only on 2 or 3. An audio pot would be halfway up
approximately to get that same level.”
“On some
of the less expensive amps, some makers use linear pots on their volume controls
so the customer thinks the amp is "loud". Imagine being in a store and trying out an amp, turning the
volume up to 2 and blasting out the store.
The customer would think that this was one loud amp, but the linear pot
makes the ears think that the amp is loud when it's just giving more volume
upfront due to the nature of the linear pot.
With an audio pot, the volume control may have to be up around halfway
to get the same perceived volume.”
We finally finished going through the various parts of the amp and all of my questions had been answered. At this point, Mikey and I had been talking for about 5.5 hours and we hadn’t played a note yet. It was time to change that!
07 - Comparing the amps
The two guitars that I brought with me were a Hamer Duotone Custom and a Hamer Artist Custom. The Hamer Duotone has a Seymour Duncan ’59 humbucker in the neck and a RIO BBQ humbucker in the bridge. The Hamer Artist Custom (my favorite guitar) has a Seymour Duncan PhatCat P-90 in the neck and a Seymour Duncan Seth Lover humbucker in the bridge.
My favorite guitar – a 1996 Hamer Artist Custom
I play a lot of different styles of music including rock, blues, country, folk and reggae. I’ve been playing guitar since 1966 and a lot of that time has included acoustic guitar, which has influenced my electric guitar technique. About 60% of the time, I finger pick with all 5 fingers of my right hand, using a modified Travis picking technique of keeping the bass going with my thumb while dropping harmonies and melody in with my other fingers.
Because of the influence of my acoustic playing, I tend to like tube amp tones that are bright and just hovering at the edge of breakup. I’ve always felt the Deluxe Reverb enhances my style of playing. I love to let open strings ring out and I like to use reverb to let the sound swirl.
I also like to get down and dirty (who doesn’t?) and enjoy playing singing leads with an edge. I also like crunchy power cords, ala AC/DC. Could one amp handle this range of styles? To get this variety out of my DRRI, I have to use a Voodoo Labs Sparkle Drive to push the amp into singing sustain.
The first thing Mikey wanted to hear was my playing style and the sounds I typically get out of my DRRI. I grabbed my Hamer Artist Custom, switched on my favorite pickup (the P-90) and started playing a variety of songs and leads. As you’d expect, even when using the humbucker on this guitar, I couldn’t coach my amp into singing sustain. Switching to the Hamer Artist Duotone and its humbuckers got me closer to this but it still wasn’t enough to push it over the top. One thing I’ve always liked about the DRRI though is the way it can give me enough brightness to overcome the darker tendencies of my P-90 and humbucker pickups. I always like some top end zing and enough crispness to let me drop into pseudo-acoustic sounds by turning down my guitar volume.
Next up was Mikey’s rebuilt amp. Using the P-90 pickup of the Artist Custom, I plugged into the Vibrato channel using the exact same settings as my DRRI. I immediately liked the smooth, complex tones that came out of the amp, which has a Weber C12N speaker. In my experience, if I don’t like a tube amp sound within the first 15 seconds, it is unlikely that I’ll be able to dial it in. Well, I’m here to say that I liked it right away!
When I plugged into the Normal channel, it was noticeably louder and had more grit than the Vibrato channel. It was also brighter, which I liked. Using the Seth Lover humbucker, the Normal channel crunches like mad and gives a very exciting rock and roll sound. Brilliant. I liked this channel even more than the Vibrato channel. This isn’t your father’s Deluxe Reverb.
The humbuckers on the Hamer Duotone have even more output and when these push the Normal channel on Mikey’s amp, watch out! Single note leads could be coaxed into singing, controllable sustain with just the right combination of overdrive and clarity, much like the beautiful lead tones that Duane Allman used. With the guitar volume at 10 and the amp volume of the Normal channel at 6, it was pure rock and roll heaven. Mikey and I were both grinning from ear-to-ear.
Mikey brought out a StarTouch A/B/Y foot pedal so that I could hear how both channels sounded together. This is a very well made and transparent pedal. I really liked it. You can check it out here:
With both channels in phase, it was easy to have the best of both worlds. I really enjoyed being able to dial-in cleans from the Vibrato channel and crunch from the Normal channel. It also made it easy to switch from one channel to the other to hear the sonic differences.
I also had a chance to try the Triode/Pentode switch on both channels. This has the intended effect of dropping the gain and making the amp darker. It’ll be nice for some applications where I don’t want to be as loud, but in general, I much prefer the Pentode setting.
Then we tried the tremolo circuit. Wow. What a difference between it and my DRRI. The tremolo has a slow, subtle, hypnotic effect that just seemed to breath. It added a lush, 3D sound similar to a UniVibe. It’s like having an effects pedal built into the amp that doesn’t mess with the pure tube tone. I rarely use tremolo but I can see myself using this circuit. Once again, I liked it right away.
Next, we plugged Mikey’s amp into the speaker in my DRRI cab. My DRRI has a Reverend All Tone 1250 (12” 50-watt) speaker that has always seemed just right for my amp. I'm not alone in my assessment of this speaker. On the newsgroups, it isn't uncommon to read that other players have found this speaker to be the ONE for a Deluxe Reverb.
I have a Celestion Alnico Blue
speaker but I prefer the Reverend All-Tone 1250 if I'm using a single 12"
speaker with this particular amp. If
you'd like some more info on this fine, reasonably priced speaker, check out
the Reverend website at:
Reverend Guitars and the All-Tone 1250 speaker
When we plugged in and fired up the Vibrato channel, the sound was darker than when played through Mikey’s speaker. It is amazing what a difference a speaker can make!
Of note, Mikey plays a customized G&L ASAT (Telecaster style) guitar, which is bright to start with. With his guitar and brighter speaker, it's not surprising that his amp played with my guitars and through my speaker was darker sounding. Since I like a bright Vibrato channel, Mikey said he’d further modify this part of the circuit for my playing style and equipment.
I would like to add that the Vibrato channel still sounded great. When I turned up the volume on the guitar using the P-90 pickup and plucked a single note, the sound from the amp swelled and receded, swelled and receded. I always view this as a critical sign of a well-designed amp. Tube amps should breath like this. It adds so much to the enjoyment of the player and the listener. This amp has it in spades.
Switching over to the Normal channel with my speaker produced a wonderful, exciting sound that was lacking nothing. Since the Normal channel on Mikey’s amp was brighter than the Vibrato channel, my darker speaker merely complimented it. Once again, I cranked up the guitar volume, switched to humbuckers and had us both grinning from ear-to-ear once again. I loved the way the Normal channel sounded through my speaker and I told Mikey I thought he’d hit a home run with this channel.
By now, we’d been at this for 6.5 hours, it was 7:30 at night and suddenly we both started to think PIZZA. We called up the local Pizza Hut and both ordered our favorite toppings. While we waited, we grabbed another couple cans of Coke and ice and sat out in the warm, night air.
After a bit, Mikey’s daughter showed up and I got to meet and chat with her for a bit. It was nice to meet a member of his family and see this side of his personality. I got to interact a bit more with the friendly cat when the doorbell rang and the pizza arrived.
Pizza
We sat outside as the sky darkened, enjoying the pizza and our further tone conversations. I was really excited by what I had just heard and Mikey was charged up by the enjoyment he could see in me. We talked for quite a while about the subtleties of tone and the qualities of the Normal and Vibrato channel through my speaker. Mikey spoke of refinements to the Vibrato channel that he could make so that it would better match my speaker, my guitars and my style.
Mikey said this type of tweaking to a player’s preference is at the core of his design approach. He pointed out that walking into a store and buying an amp, even an expensive boutique amp, doesn’t guarantee a match with a player’s guitar and style. He said he has chosen the AB763 circuit used in the Deluxe Reverb (and many other Fender amps) as a good starting point in trying to give a player customized solutions. He was proving it by me!
Mikey had a couple more things he wanted me to try, so after this relaxing pause we returned to the shop and fired up his amp into my speaker.
This time, Mikey had me play through the Normal channel while he adjusted the midrange pot on the back of his chassis. Using both guitars and all pickups, I played a variety of licks and songs, listening for the subtle and not so subtle changes he was able to elicit with this control. It became apparent that my favorite setting was right between 11:30 and 12:00 on the pot. Mikey will build a resistor into the circuit of the Normal channel of my amp to give me my preferred mids setting.
We went back to the Vibrato channel and listened again, particularly to the volume and the brightness. I told Mikey that I liked the smooth, 3D sound of the channel but would enjoy a volume level a bit closer to the Normal channel as well as a brighter sound. Of course, the Normal channel should have more gain than the Vibrato channel so that when I’m playing, I can step on the A/B/Y box and kick in with a lead sound. To get the brightness but retain the smoothness, Mikey referred again to the use of his Cap Stacking method and how he could use this to get what I was after. He made some notes and I could see that he was really interested in redesigning this part of the circuit.
Mikey suggested we try swapping some 12AX7 tubes in various stages of the amp to determine which tubes would most enhance what I was looking for. We primarily focused on the V2 position, which originally had a JJ in it. We had some nice tubes to try including the new Groove Tubes 12AX7-M (Mullard copy), a JJ, an EH and an old Telefunken. The Telefunken tube may have been at the end of its useful life. It was the noisiest of the tubes and was just so-so in terms of tone.
The GT tube was darker but added a nice swirly, lush tone. It was also the nicest feeling tube from my perspective, giving me the sense that my strings were rubber bands that I could really use to change the dynamics. The JJ was pretty quiet but was darker than the tube that I thought sounded the best, which was the Electro-Harmonix. This tube was the brightest of the bunch, had a low noise level and produced nice, swirly harmonics. When switching to the Normal channel, this was the tube that rocked out the best and gave the best, singing sustain. Even with this tube though, the Vibrato channel needed more brightness, which Mikey is only too happy to accommodate.
It was now after 10:00 so we decided to stop for the day. I packed up my guitars, left the DRRI for Mikey’s ministrations, and carried my stuff out to the car. Mikey and I stood outside for a bit talking about our day and what was to come next, the actual building of my amp. We could have kept talking for hours more but since I had a bit of a drive ahead of me, I wanted to get on the road while I was still charged up and alert. All the way home, I reflected on what a fine time I’d had at Mikey’s shop and of all of the new things I’d learned.
08 - Building the amp - Part 1
During the week after I left Mikey’s, he began to disassemble my DRRI. Every day or so, Mikey sent me digital photos of his progress. He also spent a lot of time emailing me the exact details of what he was doing and why. These have become a wonderful source of information for this article. Whenever I had a question about some aspect of the project, I’d send Mikey an email and he would patiently and thoroughly answer my questions. Here’s the first picture I’d like to share with you.
The PCBs, pots and jacks are removed during the disassembly of the DRRI
After removing the PCBs, iron, filter board, pots and jacks from the DRRI, Mikey built a brand new filter board and installed the Mercury iron. In the next photo, note that the black, phenolic octal sockets on the right that will be removed and replaced with high-quality ceramic sockets.
Mercury reverb transformer, new filter cap board, Mercury output transformer and choke
Since the Mercury iron is larger than the DRRI iron, the output transformer and choke are slightly offset. In order to keep the output transformer carefully away from the speaker, Mikey drilled a new hole in the chassis to properly position the larger output transformer.
The new filter cap board is a thing of beauty. In a quest for the best components, Mikey chose to use something other than the traditional Spraque Atom capacitors, although these are similar in appearance. They are smaller but have higher specs. For example, the Sprague 20uF caps are rated for 65 degrees centigrade whereas these can go to 85 degrees centigrade. Their tolerances are also tighter. Here’s a close-up:
Mercury reverb transformer and Mikey’s custom-made filter cap board
In this photo, you can see the high-quality workmanship on the polished board, turret installation, neat soldering and the use of the red, 18 gauge, stranded 600V Teflon wire. Mikey is primarily using Teflon wire throughout the amp. Mikey prefers Teflon wire because of the superior quality, the strands stay put when twisted and it is easier to solder because the insulation just won’t melt.
The next item on the agenda was the installation of the new, ceramic octal tube sockets. These hold the rectifier tube and the two 6V6 power tubes. Mikey chose ceramic sockets so there won’t be any chance of arcing and burning the sockets. He also reinforced the socket mounts by adding hex nuts to the socket mounting bolts, along with a dab of Locktite to make sure they stay firmly attached to the chassis. Here’s a nice photo of this section of the amp:
3 new ceramic
sockets (white)
DRRI power transformer (black), Mercury choke and Mercury output transformer
The power transformer is the stock one from the DRRI, which will be replaced next month with a Mercury Magnetics power transformer that is of excellent quality. Mikey offers this as an option.
09 - Bias Test Points
As previously mentioned, I elected to get the “bias test points” optional mod from Mikey. This does cost more but it is worth it to me. Having bias test points will let me replace the 6V6 power tubes myself using just a voltmeter or multimeter. Finding the right spot to place them in the amp without interfering with the back of the chassis is tricky. Mikey told me he was constrained by the slanted part of the back panel where it presses up against the back of the chassis as well as the rectifier socket on the inside of the amp.
Ground (black) and right and left (red) power tube test points
You can see an angled pencil mark indicating where the back panel lays across the rear of the chassis. On the inside of the chassis, there had to be room to fit the resistors between the red test points and the ground test point.
Internal wiring of the bias test points
In the photo above and the one below, you can see the bias test points are right near the rectifier tube socket. So as to avoid any problems, Mikey moved the B+ wire (red wire) from the pin on the rectifier socket so that it moves away from the bias resistors and runs along the bend of the chassis to the standby switch. It's out of the way of everything else. Mikey explained in an email to me another reason he likes to use 600V Teflon wire is that it gives an extra measure of insulation from other components.
The way the bias test points work is pin 8 of each power tube is wired to a red terminal on the inside of the bias test points. Each red terminal has a 1-ohm 1% resistor that goes to the black terminal, which is then wired to ground at one of the power transformer mounting bolts. Without bias test points, pin 8 is just wired to ground but adding this resistor allows a bias measurement in millivolts across the resistor, which converts directly to milliamps of current draw in the tube due to Ohm’s law.
Light blue wires attach at pin 8 of the power tube sockets to the bias test points
Although I’m jumping ahead of the construction process, the next picture shows the bias circuit portion of the main board created in “The Build – Part 3”. In the picture you can see the bias resistor, bias diode, bias capacitor and bias tap resistors, which create an “artificial” center tap from the power transformer.
The way this part of the circuit works is AC voltage from the power transformer enters the bias range resistor, which drops the voltage before entering the bias diode. The bias diode rectifies the AC voltage into DC negative voltage, much like the5AR4 tube does in the power section of the amp. This fluctuating DC voltage gets smoothed out by the bias capacitor on its way to the bias pot.
Bias circuit on the main board
The whole purpose of biasing is to set the right amount of current passing through the power tubes when no signal is present. Think of this as setting the idle on a car. If there’s not enough current, the tubes will sound sterile and won’t perform properly, and if there is too much current, the tubes will run too hot and there will be unwanted distortion and a good chance of tube failure.
The bias control on the bottom of the chassis is turned while looking at readings on the voltmeter, which has probes inserted into the bias test points. In this next photo, you can see the bias adjustment pot. Note that a screwdriver is placed in the white slot in the middle of the pot to turn it. As you can see, the bias pot can be turned from inside the chassis.
Bias adjustment pot as it looks from inside the chassis
What you can’t see is this adjustment can also be made from the bottom of the chassis without taking the chassis out of the cabinet. A screwdriver is simply inserted into a hole in the bottom of the chassis leading to this pot and the adjustment is made while watching a voltmeter attached to the bias test points, which are also on the outside of the chassis. Hooray. No lethal charges to fry guitar players! The voltage coming out of the bias pot goes to a junction with two resistors, which sends the right amount of separate negative voltage to the grids of each of the 6V6 power tubes. This negative voltage determines how much current goes through the tubes when they are at idle.
The bias is set to make sure the proper plate dissipation rating (in watts) of the
output tubes is reached. The maximum
plate dissipation rating for a single 6V6 tube is about 14 watts. Since it is best to operate at about 70% of
maximum plate dissipation, the bias is adjusted to bring the plate dissipation
down to about 10 watts (.7 * 14 = 9.8).
The calculation of the proper bias setting is done like this:
The formula to
calculate plate dissipation says that P = E * I, which means that Power (watts)
= Volts * Current Draw (amperes). Let’s
say the plate voltage is 400V. The
current draw (the unknown variable we need for setting the bias) can be
calculated like this:
P = E
* I
10 = 400 * I
10/400 = I
.025 = I
(amperes)
In this example, since .025 amperes = 25ma (milliamps), and because of the 1-ohm 1% resistor, the bias setting for a power tube will be right when the voltmeter reads 25mv. The next time I put in a set of matched power tubes, I would just set the bias to 25mv. When I pick up my amp, Mikey will measure the actual plate voltage (it probably isn’t exactly 400) and we’ll play with the bias to find my favorite bias setting that puts each tube close to10 watts. Since the amp uses two 6V6 tubes in class AB (push/pull), the output of the amp will be about 20 watts.
10 - Building the amp - Part
2
After the installation of the bias test points, the next step in the rebuild process is making an assembly for the pots and jacks. Since I’ve previously covered Mikey’s use of CTS and Alpha pots and Switchcraft jacks, I’ll jump right to the construction. Here’s a photo from the first batch of pictures I received from Mikey on this part of the rebuild.
Pots and jacks in the first stage of assembly
There’s a funny story that goes along with this picture. One night at about 3:00 am, I got up for a drink of water and decided to check my email. Mikey must have been working late that evening because I’d received a new batch of pictures. When I opened the email and saw this picture I knew something wasn’t right. Why were the jacks and pots on the outside of the chassis? Why were they upside down? What happened to the black faceplate? Was I losing my mind or was something else going on? I emailed Mikey looking for answers. Here’s what he wrote back.
“Hey
Wayne! No, you're not crazy. You were just half asleep and you were
looking for something that wasn't there!
I took the faceplate off after I enlarged the holes. The faceplate has to be on when enlarging
the holes for the CTS pots and input jack insulators so the faceplate holes
will be enlarged as well. Then, I take
the faceplate off and store it safely somewhere else while I'm working on the
pot buss assembly. The faceplate is
held on the chassis by the pot nuts, input jack nuts and the pilot light
assembly. It's just a piece of metal
with holes in it. When the pots and
input jacks are put through the holes, and the nuts are attached and tightened,
that's what holds the faceplate on.”
“I temporarily
mount the pots on the outside of the chassis, upside down. This gives me a better work angle, plus,
when I solder the buss wire to the back of the pots, the pots turn out
perfectly spaced, since I'm soldering the buss wire with the pots in their
correct holes. When completed, I
unscrew the pots from inside, lift the assembly and just put it back through
the holes from the inside of the chassis.
Now, the pots are right side up, the spacing is perfect, and life, as we
know it, is good!”
Whew. I felt better after hearing this! My brain was still intact and I was curious about the “buss wire” he mentioned. You can see it in the pictures. It is the silver, unshielded wire connecting all of the pots in each section. It is used to ground the pots, which are part of the preamp section. Here’s a close-up shot.
Buss wire soldered to the backs of the pots
Soldering the buss wire to the pots isn’t as easy as you might think. Mikey explained he uses a 100 watt soldering iron to heat up the area enough to make sure to get a nice even bead over the buss wire. This buss wire and other grounds from the preamp section are kept separate from the grounding of the power amp section. Since the input jacks are isolated from the chassis by the insulating shoulder washers, the input jacks can be grounded to the same ground used for this buss wire. Here’s a picture of one set of the input jacks.
Input jacks for the Normal channel
If you look carefully, you can see that Mikey uses twisted pair, insulated Teflon wire for the input jacks (the white wire on the right). In addition to the two wires from this twisted pair, there is also a metal-mesh shield inside the Teflon outer shell. Mikey attaches a black grounding wire to this shield. It is wired to the appropriate parts of the input jacks to keep hum and noise from entering the input wires.
You can see the black grounding wire going from the input jack over to the beginning of the buss wire on the back of the pot, grounding the jacks and the Teflon input wire. The other end of the white Teflon wire will go to 68K resistors, which will be attached directly to the input grid of the preamp tubes. By the way, did you notice the pots and jacks are back inside the chassis where they belong :>)
Next up is the creation of the main circuit board that will house the majority of the resistors and capacitors used in various stages of the amp. Before doing this article, I had no idea how much work goes into the creation of a custom turret board. Of course, the most critical part is getting the circuit design done correctly. With all of the optional mods I’ve selected and the enhancements Mikey has added since the rebuild of his personal DRRI, there are quite a few circuit changes. For this step in the process, the design changes are added to the existing design template.
Custom turret board template showing many design changes
The 1/8” G10 board is cut to size from a larger piece of G10 material and the edges are sanded smooth.
Sanding the edges of the newly-cut G10 board
Here’s a photo of the three items needed to create the custom turret board.
G10 board
(bottom), circuit template, metal drill jig (top)
The G10 board is placed on the drill press; the metal drill jig is placed on top of it, followed by the circuit template. This is all well secured to the drill press in preparation for the actual drilling. Wherever there is an open hole or an indicated spot for a new hole on the template, a hole needs drilled through the G10 board. The jig provides a secure path for the drill bit so the holes are cut cleanly on the board. It is time consuming to drill all of these holes.
Drilling the G10 board (underneath and out of site) through the template and metal jig
Completed G10 board just after removal from the drill press
Once the drilling of the board is complete, Mikey sands and polishes it. This is the last step of the board preparation before the installation of the turrets and board components.
Completed and polished G10 board
11 - Building the amp - Part 3
With the completion of the drilled and polished G10 board, the next step is the installation of the turrets. As you will recall, a turret is placed into a hole in the board. A special tool is put on the drill press and this is pressed into the turret in the board. As the drill press turns this attachment, an expansion of the base of the turret inside the board creates the gripping pressure to assure the turret is sturdily secured. This time-consuming process is continued until all of the turrets have been installed in the G10 board.
Turrets are seated into the board using this special drill press attachment
The finished G10 board with all of the turrets securely in place
Take a look at the top left corner of the completed turret board and you will see an angled cutout. Mikey did this to allow space for proper clearance of the CTS 10K bias pot that will be installed into the bottom of the chassis directly below this area.
With the turrets in place, the power rails and ground rails are installed. Mikey has come up with an innovative way of doing this instead of using the typical, un-insulated buss wire. The red and black wires in the picture look like separate pieces of Teflon stranded wire but they are not. Instead, he takes a single length of 18-gauge buss wire (the same kind soldered to the back of the pots) and runs one continuous piece to the power rail turrets and to the ground rail turrets. Just before soldering these in place, Mikey removes the Teflon insulation from some 18-gauge stranded wire, cuts it to size and slides the Teflon shielding over the 18-gauge solid buss wire.
The power rail (red) and ground rail (black) buss wires are soldered to the turrets
In this close-up, you can see the how the continuous buss wire, wrapped in Teflon shield is wrapped around the turret and soldered. Also notice some of the turrets are wired together with thinner buss wire than what is used for the power and ground rails. Having the turrets wired together performs the same function as hooking up two components to the same terminal, which will be done in a few places in this amp. It’s really the choice of the amp designer. Doing it this way keeps the components lined up nicely but it requires additional labor to install the extra turrets.
Power rail (red) wrapped around turret
From this point, there is a fair amount of work that goes into putting the hookup wiring into the rest of the board. In order to minimize noise, Mikey runs the signal wires under the board, surfacing them right at their connection points. In the next photo, you can see the DRRI chassis with the pots and transformers installed, awaiting the completion of the main board.
DRRI chassis (top)
and main board with all hookup wires (below)
With the hookup wires in place, the next step is the installation of the components, primarily capacitors and resistors. In an earlier section I discussed the construction of capacitors. Remember, the function of capacitors is to restrict DC current and shape tone. The function of a resistor, as the name implies, is to provide a specific amount of opposition to current flowing through it. This causes a voltage drop, which the resistor does by radiating heat. This lets you control how much voltage you want to work with in various parts of the circuit. Resistors have a power rating that tells how much power/wattage (volts x current) they are capable of handling.
In the stock DRRI, some1/4-watt resistors are used, which Mikey feels are undersized. He uses 1-watt resistors throughout his circuits with the exception of a few 1/2-watt metal and carbon film resistors in non-critical sections. Of interest, the pots used to control the amp are just variable resistors. You can identify resistors very easily in the photos since they have bands of color, which are used to show the rating of the resistor measured in ohms. The rest of the components soldered to the turrets are capacitors, which come in a wide variety of colors, shapes and sizes.
Capacitors and resistors in the Normal channel of the amplifier
This next photo is of the “Vibrato” channel. For those of you that might not know, Fender created a lot of confusion by naming the second channels of their amps “Vibrato”, which is the modulation of pitch (like what a whammy bar on a guitar does). In fact, it is Tremolo, not Vibrato, which is produced by this part of the circuit. Tremolo modulates volume, not pitch. Fender knows better but they continue to use the word Vibrato on their amps so I have chosen to refer to this channel as Vibrato during this article because that is what it says on the faceplate of the DRRI.
Capacitors and resistors in the “Vibrato” / Tremolo channel
Close-up of
capacitors (red), resistors (color banding) and wiring
As I examined the pictures Mikey was sending me of the building of the board, I could see the red wires were connected to the red power rail and the green wires were connected to the black ground rail. I assumed the white wires were signal wires (carrying the guitar signal) but I wasn’t sure about the blue wires so I asked Mikey. Here’s what he said:
“The red wires do go to the power rail and the
plates of the preamp tubes. The greens
do go to ground and go to the cathodes on the preamp tubes. The white wires are signal wires. On the bottom side of the board, they go to
the grids of the preamp tubes and on top, they'll go to the pots. The blue wires run from the phase inverter
to the power tubes so they're like signal wires. I just use blue to
differentiate those, plus, I like blue!”
Fully
completed main board with all components and hookup wires in place
Left
side of completed main board
Right
side of completed main board
12 - Building the amp - Part 4
Now that the components and hookup wires are installed on the main board, it is placed inside the chassis.
DRRI
chassis with the main board installed
Here’s a close-up shot of part of the chassis interior with some features of the amp highlighted.
Close-up
of chassis interior during the later stages of assembly
(Note
the soon-to-be-replaced DRRI power transformer)
The phase inverter (also known as a phase inverter splitter) is a very important circuit in the amp. The input from all of the previous sections goes through a mixing resistor and then to a coupling capacitor that links this mixed signal to the phase inverter (PI). The phase inverter circuit, made up of resistors, capacitors and a 12AT7 tube take the incoming signal, split it in two, and then changes one of the signals to be 180 degrees out of phase with the other.
The idea is to have one of the 6V6 power tubes amplify one half of the sine wave of the incoming signal and the other 6V6 amplify the opposite half of the signal. This is known as running the power tubes in push-pull mode, which is also called Class AB. While one tube is amplifying its part of the sine wave, the other tube rests. Class AB operation is able to generate more power out of the two tubes than if they were both running all the time, which is called Class A. Here’s what the phase inverter section of the main board looks like.
Layout
of the phase inverter and NFB loop on the main board
Follow the numbers in the photo to see how this circuit works.
1. The blue wire to the top right of this resistor brings the signal from all previous circuits in the amp into what is called “the mixing resistor”.
2. This passes through the coupling capacitor (.002uF Mallory metallized polyester) into resistors and capacitors in the phase inverter circuit.
3. The two big, yellow capacitors (and two caps hidden underneath them – Cap Stacking) handle the split signals. They each connect to the plates of the 12AT7 phase inverter tube via red wires, which can’t be seen here but they are visible in the next photo. The blue wires at the bottom of this photo connect to pin 6 of the power tubes, which is connected to pin 5 (input/grid) via a 1.5K swamp resistor.
4. Negative Feedback Loop (NFB). After the phase inverter tube, the 6V6 power tubes amplify each half of the sine wave and send the amplified signal to the output transformer to drive the speaker. The white wire (#4) is coming back from the positive terminal of the speaker output jack into two resistors that feed a part of the output voltage back into the phase inverter.
5. Take a look at the red capacitor on top of the resistor (#5). This cap acts like a fixed presence control in the NFB circuit.
Whew. That’s a lot to think about! It took me awhile to understand how the phase inverter and negative feedback circuits work. I spent about an hour on the phone talking with Mikey about these circuits and why he chose the specific types of resistors and capacitors used here. I also went to my bookshelf and looked up these circuits in a couple of my tube amplifier books to make sure I could explain it reasonably well. While I’m thinking of books, you might like to know about the books and videos I have in my library to help me with my hobby. You can find the whole list in the “Contact Information” section of this article.
Let’s take a moment to diverge from the technical discussion so that I can share some of Mikey’s philosophy on choosing various components and circuit modifications in an amp. He told me on the phone he views it much like cooking. You start with a good base (like the AB763 circuit in the Deluxe Reverb) and add a pinch of this and a dash of that to bring out complexity and character in an amp. Just as one might add lemon to a sauce to balance out the sugar, Mikey says he likes to make subtle tweaks to guide the amp to the desired sonic result.
Going back to the technical stuff, this “amp cooking” is very evident in the phase inverter and negative feedback sections mentioned above. Take for example the coupling cap after the mixing resistor. The standard capacitor used here in the DRRI is .001uF, which is one of the reasons the stock amp is very bright. In Mikey’s personal amp, he changed this to a .0047 capacitor, which is about 5X more than stock. This emphasizes the mids and upper mids, adding some phatness to the tone, which works great with Mikey’s G&L ASAT guitar. But since I expressed interest in a brighter sound from the amp on the Vibrato channel, Mikey has put a .002 metallized polyester coupling cap in my amp, allowing it to be brighter than his amp while giving more of the upper mids than the stock DRRI.
But just like adding lemon to balance sugar, Mikey wants to make sure there is enough brightness in the amp for my tastes so further modifications are made in the PI circuit. For example in my amp, the two big yellow caps (#3) are Mallory-150 .047 metallized polyester caps for a smooth tone but he has used his Cap Stacking method to add two .01uF metallized polypropylene caps (white) right underneath them. Together, they have a rating of .057uF. Here’s a close-up of the Cap Stacking.
Cap
Stacking using .047uF caps (top - yellow) and .01uF caps (bottom – white)
Metallized polypropylene caps (bottom – white) tend to bring out some brightness. This should help balance the .002uF coupling cap to help me get the brightness I’m looking for. Another tweak to the recipe that Mikey is using is the placement of a capacitor in the negative feedback loop (#5), which acts as a fixed presence control to help give some sizzle in the top end of the tone. The goal is to have an even response in the lows, mids and highs with a bit of personalization for the player. As you can see, a pinch of this and a dash of that make for a tasty dish, personally tweaked to my sonic preferences. Can you dig it? I can!
There are only a couple of items to finish on the build. All the remaining hookup wires are connected to the pots, jacks and switches. The Triode/Pentode switch gets installed and all of the tubes must be powered using filament wire. If you glance back at the pentode circuit diagram in the “Design Details” section, you can appreciate that the filament wires are connected to the cathodes of every tube. This heats up the cathodes so they start emitting electrons.
Completed
connections on the almost-finished chassis
Take special note of the large. blue resistors between the bias circuit and the phase inverter circuit on the board. These are the 1K/5W screen resistors used to protect the 6V6 powers tubes. This is how this whole project started in the first place! The 470-ohm/1-watt screen resistors kept burning up when a 6V6 tube failed. Now, if a 6V6 tube fails at the screen grid, these 1000-ohm/5-watt resistors will remain intact and I shouldn’t need to have the amp serviced. I can just pop in a new matched pair of tubes and set the bias using the new test points.
I find it amusing my initial desire to replace these two weak components has led me along the path of getting this entire amp rebuilt :>)
13 – Mercury Magnetics
As I mentioned earlier in the article, I had not originally intended to use a Mercury Magnetics Power Transformer (PT), electing instead to save a little money and use the stock DRRI PT. I’ve since changed my mind. How this came to be makes for an interesting story.
About 10 days before I was scheduled to pick up my amp at Mikey’s shop, I sent out emails to all the companies that I linked in this article. This included Mercury Magnetics since Mikey was putting their output transformer, reverb transformer and choke in my amp. Shortly after sending out an email to Mercury, I heard back from Paul Patronete who graciously gave the OK to link to their site. He also took a moment to let me know in addition to the other Mercury iron I was using, their power transformer would make an improvement in the tone of my amp. Paul invited Mikey and me to give Sergio Hamernik (an owner of Mercury Magnetics) a call to discuss the benefits of the power transformer.
I really appreciated the helpful suggestion but I hadn’t ever heard of a PT making a difference in tone, thinking the other pieces of Mercury iron would make 90% of the improvement. I did let Paul know I’d add a comment in this article saying I’d heard from Mercury and the PT would make a big improvement in the tone. He wrote me back with an interesting bit of information I had never considered. Here’s a paraphrase from the email he sent me:
The stock reissue PT does an OK job of
supplying the required voltages to the circuit however the overall PT's
behavior under working conditions is not the same as the original vintage
Deluxe Reverb PT. Characteristics that
make the reissue PT different than the original PT involve flux density, AC
regulation and primary inductance. These characteristics are a
very important factor for the player that wants to nail the tonality
of the vintage Blackface Deluxe Reverb amp.
Paul once again encouraged me to talk to Sergio. This certainly piqued my interest so I got in touch with Mikey and discussed Paul’s comments. We decided to get Paul on a conference call and to also see if we could speak to Sergio. We called Mercury in the late afternoon, four days before I was due to pick up my amp. Although Sergio wasn’t in, Paul spent almost 1.5 hours talking with Mikey and me about their power transformer and the rest of their iron.
Paul is a guitar player first and foremost and his enthusiasm for great tone is infectious. In addition to talking about all the good things Mercury Magnetics has brought to the industry and guitar players, he talked about what some of the cutting edge builders are doing. Mikey and Paul had very similar opinions on what makes great tone and why. It was a very informative and fun conversation. At the end of it, I certainly wanted to speak to Sergio so Paul invited Mikey and I to call again in the morning.
The next day, Mikey and I called Paul and after a brief chat, he brought Sergio to the phone. It was truly a treat to talk with Sergio and I learned a LOT about tube amps, the importance of an amp’s magnetics (the iron) and primarily, the benefits of using a Mercury Magnetics power transformer engineered for a Deluxe Reverb. As we talked, I jotted down as many notes as I could manage. I’ll share them with you now, although please keep in mind I may not have captured everything Sergio was conveying. He did a fine job though of keeping his information geared to my level of understanding.
Sergio started by asking me if I knew what modulation is. Of course I’ve heard the word but I wasn’t sure what it meant in relation to tube amps. He explained that an amplifier is a modulated power supply whereby the guitar signal is used to modulate the amp and so the quality of the power transformer affects tonality. He said the power transformer (along with the rectifier tube, a 5AR4 type) is responsible for taking the AC wall current and transforming it into the DC current used in certain parts of the amplifier. He said the power supply sets everything else up and everything depends on the character of the power supply. For example, since it helps determine the AC and DC voltages going to each section of the amp and tubes, it has a big impact on the feel of the amp.
I learned that although the stock DRRI power transformer provides the right amount of volts and amps (current) to the amplifier, the DRRI power transformer is nothing like one of the original PT’s used in Deluxe Reverbs. Sergio said among other things, the flux density and primary inductance all have to be right. As a comparison, the DRRI stock power transformer has 1/2 or less inductance on the primary than a Mercury Axiom PT. (You may recall from a previous section; inductance is the opposition to changes in current.)
He said capacitance and leakage reactance are critically important and Mercury Magnetics power transformers have lower flux density and heavier iron with better permeability. Capacitance and leakage reactance between the windings, and where the filaments are in relation to the B+ supply, really matters. Now that’s a mouthful! I think I’ll need to spend some time researching the concepts mentioned in just the last two paragraphs so I can better understand the electrical theory behind his designs. One of the best things about this project and writing this article is being exposed to new concepts. This is my idea of fun!
Sergio did break this theory down into practical examples for me. He explained the Mercury Magnetics method starts with finding the very best sounding amps and examining their iron. For example, if there is a great tone on a recording, Sergio can try to find the exact amp used on the recording and use it as the model for new iron.
Mercury
Magnetics Axiom Power Transformer for a Deluxe Reverb (not for a DRRI)
The DRRI power transformer has different physical dimensions than the one in a Blackface Deluxe Reverb. Mikey has an electronic caliper that does very precise measurements so he sent these dimensions to Paul so Mercury can provide the perfect power transformer for this amp. They have the know-how to build power transformers that will sound just like one in a Blackface Deluxe Reverb. They do this by using the very best example of an original 1965 Deluxe Reverb transformer, unwinding it and laboriously documenting every nuance of how it is constructed. Then a combination of old and new technologies is used to create the very best transformer possible.
There’s a reason why guitar players seek out older amps, and this obviously is because of better tone, which I’ve come to understand is largely due to quality magnetics. I learned the magnetics available to amp builders in the 50’s and 60’s were made from materials optimized for the military for WWII and the Korean War. There was a tremendous overstock from the military build-up and amp builders had ready access to very high quality materials.
Modern materials are designed to be cheaper to hit price points. Sergio told us he specs custom steel to make Mercury transformers. They do enough volume of business that he can insist his steel manufacturer hand crank the steel through a gas oven, just like it was done in the 50’s and 60’s. Steel and copper behave differently at different milliamps so the properties of the metal must be designed and manufactured to this end. For the guitar player, this level of quality shows up in things like the ramp rate of note attack, the feel of the pick attack and better bass note response. He also used the choke as an example, where the Q-Factor (the measure of quality in a resonance system) is quite a bit different between the stock DRRI choke and the Mercury choke, so much so that it makes for an audible difference.
Sergio explained the power transformer is rated for a lot more watts than the amp produces. For example, the Mercury Magnetics Axiom power transformer for my amp will be rated at 120 watts and is capable of supporting an audio output of up to 60 watts. The general rule-of-thumb is the power transformer needs to be rated at twice as much as the maximum audio output.
Since this power transformer has specs well in excess of what the two 6V6 tubes are capable of creating, it can safely run 6L6 tubes. 6V6 tubes require only 60% of the power of 6L6 tubes. The output transformer is also capable of handling 6L6 tubes so by getting the Mercury Magnetics power transformer, I can get more power and a different tonality out of my amp than with the 6V6’s. This is a nice bonus and one I hadn’t considered when Paul first emailed me, suggesting their PT. I prefer 6V6’s in the amp but it’s nice to have another option.
Mikey asked Sergio to describe the sonic differences we might expect with the use of the Mercury Axiom reverb transformer. Sergio said the sound differences are subtle because it is such low power but if you have good ears, you’ll hear differences in “evident detail” and a “perceived wider bandwidth”.
Mikey and Sergio talked a bit about the dramatic effects of using the Mercury output transformer. Sergio likened his output transformers to a window with no glass and no screen, allowing all of the tonality of previous sections of the amp to pass through with great clarity. He explained the excellent “phase relationship tonality” (perceived as a 3 dimensional swirl) is dependent on the quality of the construction of the transformer. By comparison, the DRRI output transformer acts as dirty glass covered by a dirty screen. I couldn’t help but be reminded of the scene from the movie “My Cousin Vinny” where Vincent Gambini (Joe Pesci) disproved the testimony of a witness by pointing out with pictures how the witness was looking through dirty glass, a bug covered screen, and through trees and bushes at a fleeing car.
As a tube amp enthusiast, I’ve been reading and learning about tube amp electronics for quite some time. In this conversation with Sergio I learned a lot of things I had never considered. Of course, by now I was completely convinced their power transformer should go in my amp! You can understand why the top amp builders call upon Sergio for his expertise, designs and remarkable archive of information.
If you take a look at the Mercury Magnetics site you will find a large selection of iron to fit just about every need.
Sergio and Paul both told us there were very significant differences in iron on the same models of amps from the same era. Sergio spoke of a conversation he’d had with Leo Fender about amp components. Leo told him the Fender Company experienced a swelling of demand for amps when young people became enamored of guitar. Fender was just trying to meet the enormous demand so they put out specs for things like power transformers to various suppliers. The suppliers had a lot of latitude in how they meet the specs, creating a lot of variation in the parts.
Here’s another new bit of tube amp knowledge I learned from Sergio. He told me all magnetics (iron) in an amp need to be broken in over a 20 to 40 hour period of playing time. He said initially, there’s a slight harshness in the tone, which can also be characterized as a lack of sweetness. The magnetics have to be thermally and magnetically cycled. Apparently, it is similar to breaking in a speaker. Paul later told us he’s had guitar players call him up after a month to describe the new subtleties and complexities emerging from the amp: a wonderful and unexpected surprise.
After spending about 45 minutes on the phone with Mikey and me, Sergio turned us back over to Paul. I must say it was a pleasure and an honor to speak with Sergio and he couldn’t have been more gracious in taking his time to explain all of the information I’ve relayed above.
Paul and Mikey made arrangements to take the exact measurements of the bolt placement and chassis openings in the DRRI for a new power transformer. I sat back for a bit as Paul and Mikey continued where they’d left off the night before, talking about new amp ideas Mikey has been considering, which will involve creating an amp using a wide variety of power tube options. It’s too early to go into details about that though. I’ll have to save that for another article :>)
By the way, we got to know a bit more about Paul Patronete. He is a key associate of Sergio who talks with guitar players and amp builders all the time, helping people decide which Mercury Magnetics iron is right for them. You can tell he really gets off on helping players attain tonal nirvana.
Of interest, before coming to Mercury Magnetics, Paul was the General Manager for Groove Tubes for many years. I dug out my copy of Aspen Pittman’s “The Tube Amp Book – 4th Edition” and sure enough, there is a picture of Paul on the inside of the back cover. He’s also listed in the very front of the book in the “Strokes For My Folks” page and a color photograph of his Orange amps is also in the book.
Well, I’ve gone through all of my notes from my discussions with Paul, Sergio and Mikey so I guess it is time to close this chapter. I’ll be picking up my amp tomorrow although it will have the stock DRRI power transformer in it until a new one is manufactured. I sure am glad Paul took the time to talk to me, educate me, and with Sergio’s help, convince me to get a Mercury Magnetics power transformer to go along with the rest of the Mercury iron in my amp. I’ll make another trip to Mikey’s shop in Baltimore next month and I’ll be sure to let you know how it sounds, so stay tuned.
14
- Another visit to Mikey’s shop
On a Saturday in late August, I traveled once again to Mikey’s shop in Baltimore to pick up my completed amp. I don’t have to tell you I was pretty excited on the drive from my house. This was going to be the first time I’d get to see the completed main board installed in the chassis and hooked up to the rest of the components.
I arrived at 1:30 and spotted the friendly cat peering out the window at me as I walked up to the front door. Mikey thought it would be enjoyable to sit out back and talk about the project we’d both been working on for seven weeks. Of course, he’s been doing the building and I’ve been doing the writing. You may be surprised to know I’ve put more than 150 hours into crafting this tale and creating this website. It’s a labor of love though, since I’m the one benefiting the most by having to explain what I’m learning.
We grabbed a couple of soft drinks and as Mikey was getting the ice and glasses, I was able to make peace with the other cat that was recalcitrant on my last visit. The cat’s name is Simba and at first, he scampered away when I came too close but I was able to coo and woo him into accepting my ministrations of petting, although he didn’t seem to think too highly of the whole process.
Mikey and I settled in deck chairs on the patio and spoke of all the work leading up to this point in time. He said when he completed the chassis last night, he fired it up to make sure everything was performing properly, using his tubes in my new amp. He said he was going to have me install and bias my tubes as an inaugural gesture.
After our conversation, we went to the shop to look at my amp.
Completed chassis, just waiting for me to try it out
While I had learned a lot from the pictures and emails from Mikey during the build, and from researching concepts I wasn’t familiar with, I still had some questions about tube amp theory. This was a great time to get some questions answered and Mikey obliged. He gave me a guided tour of the interior of the amp using a wooden chopstick (to prevent shocks). While most of the amp’s sections are already documented, here are some of the new things I learned.
Perhaps you’ll recall in the “Bias Test Points” section, I made mention of Mikey measuring the plate voltages of the 6V6 tubes in order to calculate the right bias setting. He hooked up a multimeter, which showed the left tube at 401.8 volts and the right tube at 402.5 volts. If you look back at the formula, you’ll see the 25mv setting at the bias test points will be just fine with these plate voltages. However, these plate voltages will likely change when the new Mercury Magnetics power transformer is installed next month so this will be checked again at that time.
Speaking of power transformers, talking with Paul, Sergio and Mikey the other day about the new power transformer sparked a new level of interest on my part about this important aspect of tube amps. It’s funny, I’ve read the text books on power transformers and looked at the diagrams but it wasn’t until today that I feel I finally have a grasp on how it does what it does and why it is so critical. Maybe as consumers we are used to plugging into an AC outlet in the wall and getting power that we may tend to think of as not-too-complicated and very straightforward. I’m sure to an engineer, my next statement seems like a “duh” moment, but the power transformer does its magic by stepping voltages up or down depending on what is needed.
I’ve known from the textbooks that transformers are made from coils of wire wrapped around a magnetic core and when you place a coil of wire next to another one, a signal in one coil is “induced” into the neighboring coil(s). There are wires going into the first coil (called the primary) and wires coming out of the second coil (called the secondary). If the number of turns of wire in the primary and secondary are the same, the voltage will be the same in both. If there are twice as many turns in the secondary, the voltage will be double what it is in the primary.
As an example, if you take 120 volts AC coming from the power switch (which is connected by the AC power cable to the wall outlet), the secondary will have 240 volts of AC on it. Voltages can be stepped down in the same way by putting fewer turns of wire on the secondary coil. And guess what, there are multiple secondaries in the same power transformer.
What Mikey helped me with today was actually looking at where the connections to and from the power transformer are made in the amp. I think this really helped me visualize and understand what is happening in the amp in a practical sense, not just as theory. I got a lot of my questions answered, although as I am writing this, I needed to pick up the phone for some additional help from Mikey. We looked at photos of the chassis and he referred me to an excellent diagram on Doug Hoffman’s website:
Hoffman AB763 board layout (click on picture for an external link)
You’ll notice this layout is different than the Fender Deluxe Reverb AB763 layout shown previously in the “Rebuild Discussions” section. This is because Doug did a redesign of the layout. It is this layout Mikey has used as the foundation for my amp, although he certainly has added a number of enhancements and mods. Since this diagram is very close to my amp, Mikey used it to help explain more about the power section of my amp including the power transformer, rectifier, standby switch, filter caps, choke and dropping resistors.
Another diagram I found helpful is this current flow diagram from Doug’s site. Although it is for a Plexi circuit instead of the AB763, it has arrows showing how the currents are flowing through the various sections of the amp.
Hoffman Current Flow Diagram (click on picture for an external link)
In trying to learn and write about these sections of the amp, I’ve found myself going back to Doug’s site a number of times today. It is chock full of information and diagrams to help people like me understand what our amps are doing. For convenience, here’s a link to his library of information.
Hoffman Amplifiers Library of Information
With my newfound knowledge, I can tell you the power transformer takes the 120 AC volts from the wall outlet and has secondaries to step it down to 5 volts AC to heat the cathode of the rectifier tube and to 6.3 volts AC to heat the pilot light and the cathodes of the rest of the tubes.
As you’ve read, a lot of the amp runs on DC power (just like the batteries in your flashlight), and this has to be created by the power transformer and the rectifier tube. There’s a secondary from the power transformer to carry the AC current to the 5AR4 rectifier tube, which converts AC into pulsating DC. This high DC voltage (more than 400 volts – sometimes called B+) goes to the standby switch and then to big electrolytic filter caps that smooth out the rippling/pulsating DC power into smooth DC power. Take another look at the filter cap board:
Filter caps 1
& 2 in parallel
Filter caps 3, 4, 5 for other, individual amp sections
The pulsating DC coming off of the rectifier tube goes into the parallel filter caps (1 & 2) to smooth and filter the pulsating DC. The output transformer is fed off of these two parallel caps. The now-smooth DC voltage flows from these caps to the choke, which as you’ll recall is an inductor that has the property of opposing changes in current, so, the choke smoothes the current. Of interest, even when you turn off the amp, the current in the inductor doesn't stop immediately. It takes longer for the current to stop in the inductor because of the nature of the inductor and its opposition to changes in current.
The current continues on the power rail to filter cap 3 on the way to the 1K/5W screen resistors.
Current simultaneously passes along the power rail through a dropping resistor (to lower the voltage) and then through filter cap 4 on the way to the phase inverter. It also passes further along the power rail to another dropping resistor and filter cap 5 to provide smooth power at lower voltage to preamp sections. Here’s a part (edited by me) of Doug’s AB763 layout showing the placement of the power transformer, rectifier, standby switch, filter caps, choke and dropping resistors. You can match the layout up with the actual photo of the filter cap board.
Blow-up of the Hoffman AB763 layout with numbered filter caps and dropping resistors
Well, I think this is about as far as I can take my explanation of the power supply section of the amp. It sure helps having Mikey to talk to and a great site like Doug’s to visit for the diagrams and further explanations. I really learned a lot in the last two days. I’ve just scratched the surface, but if you want more info, it’s out there.
After Mikey and I got through discussing the power section, we needed a break so we commandeered our haunt on the back patio, drinks in hand. About this time, one of Mikey’s sons stopped by and I got to meet him briefly. We sipped our Cokes and thought aloud what the reactions to this website might be. Neither he nor I am aware of a customer having gone to this much detail about a builder’s work.
We’ve seen websites of people building David Allen’s kits, pictures from the AX84 projects, and of course photos from all of the regular and boutique amp builders’ sites, but Mikey and I seem to have struck on a symbiotic relationship that will show folks what he’s up to, while simultaneously helping me get some education. Believe me, it’s been a LOT of extra work on Mikey’s part to feed me photos and information during this rebuild but I know he’s getting a kick out of having the whole process documented.
By this time, I’d been at Mikey’s for about 3.5 hours and it was time to return to the shop for the christening of my amp. Mikey took all of his tubes out of my amp and we lined up the tubes I’d had in my DRRI before the rebuild, except for the power tubes, which are the new JJ 6V6’s I’d just received from Lord Valve. I’ve been curious to try them out. I carefully inserted all of the tubes into the amp. The moment was at hand.
The rebuilt amp is fitted with tubes and is ready to fire up
Once all the tubes were installed and the speaker plugged in, I flipped on the power switch and let it warm up. I hooked up the multimeter to the bias test points and flipped on the standby switch. It took just a minute to bias the 6V6 power tubes to the 25mv setting and I was ready to play.
I plugged in my favorite Hamer Artist Custom and beautiful tones erupted from the Normal channel. We switched to the Vibrato channel and it was excellent too.
I was curious about the Triode/Pentode switch so I put the amp on standby (something you should always do when using this switch) and put it in Triode mode. The amp was a bit darker and it growled at a lower volume. Pretty neat. And my-oh-my, Mikey certainly had come through with enough brightness, as I’d hoped. Even in Triode mode with a neck P-90 pickup, there was plenty of top-end to satisfy me with the amp’s treble setting on about 7.
Naturally, the amp sounds different when it is out of the chassis so I shut the amp down and Mikey put the chassis into my cabinet and hooked up the reverb pan to the new RCA jacks at the bottom of the chassis. Just before he attached the back plate, I snapped a photo:
The back of my amp without the back plate. Note the Reverend All-Tone speaker.
After the amp was buttoned up, it was time to give it a workout. I was astonished at how smooth, snappy and swirly the Vibrato channel sounded. It made my pulse quicken. My first thought was this is the great tone I’ve heard in the blues clubs of Chicago. My company’s headquarters are in Chicago (I’m a printer by trade) and when I get out there, I spend my evenings in the blues clubs. Depending on how I dialed in the amp and my guitar, I could go from a swampy, smoky sound to the brilliant tones of Albert Collins. I like a nice, crisp attack to the notes and the Vibrato channel has got it.
Then I turned on the reverb with the Mercury transformer and the new audio pot and once again, I was just in love right away with the sound; always a good sign. I switched over to the Normal channel with the more Marshallesque tone and let it rip. I was crunching and sustaining to beat the band. Both the P-90 and Seth-Lover humbucker sounded great. The tremolo is hypnotic. Much better than the on/off circuit in the stock DRRI. I’ll use this to add a subtle swooshing sound for some songs.
I’m not going to go into a lot of my tonal impressions in this section since I will continue to document my impressions in the “Playing The Amp” section as time goes by. Let me just say my first impressions of the amp are that IT ABSOULUTELY SOUNDS FANTASTIC!!!!!
I relinquished control to Mikey who took a few sonic excursions. It was nice being able to focus on the sounds of the amp and not my playing. I got goose bumps. I got chills. My endorphins were souping me up. I was excited, satisfied and really, really pleased. I could see Mikey felt the same. Hip, Hip, Hooray!
While Mikey continued to play, I went over to my kit bag to make out a check for the amp. I don’t recall it ever being so easy to part with a chunk of dough for gear. I kind of felt sorry for Mikey because I was going to be the one to take it home and enjoy it. LOL. Of course, he has his own rebuild so he won’t really be roughing it.
Mikey stopped playing and I handed him the check and thanked him profusely. Then he surprised me. From the back of the workbench he pulled a small, gift-wrapped package and handed it to me. I tore off the wrapper to find an Amperex Bugle Boy 12AX7 to use in my amp. What a thoughtful thing to do. Here’s a picture.
Amperex Bugle Boy 12AX7 preamp tube
If you look closely, you can see it still has the “Factory Sealed” tape holding the box closed. This photo has generated an interesting discussion on The Gear Page regarding the origin of this tube and box since it says “Made in USA” but Amperex tubes were manufactured in Holland. I’m going to find just the right spot for it in my amp.
About this time, Mikey’s wife came home from the pool. I packed up my stuff and then had a chance to talk with her for a little bit. I told her how pleased I was with the amp. We chatted some more and then Mikey and I sat out on the patio to unwind. It took awhile. We were both jazzed and amazed. The amp met our highest expectations. We marveled at the journey we’ve both been on and we’re both looking forward to having a little more free time for a week or two :>)
I’ve got a gig on the Saturday of Labor Day weekend so that’ll be the first public appearance of the amp and I’ll make sure to give a sonic report from the experience. I’ve got that following week off and I’m going to the shore for some relaxation. I expect the first time people will see this article is over Labor Day weekend. I should have everything on the website ready to go by then, so if you are reading this, mission accomplished. LOL.
Mikey and I ended up talking for a couple of hours and as the sunlight started to fade, I packed up my new amp and started the drive back to Lancaster County, satisfied and full of expectations. When I got home, I plugged in and played for a solid hour. You can read about it in the next section.
I’ll close by mentioning the rebuild isn’t done yet. Remember, Mercury Magnetics is making a new power transformer for Mikey to put in my amp. A week or two after I get back from the shore, I’ll go back to his shop to have it installed. Heck, by the time the new power transformer breaks in, it’ll be well into October before I’ll be able to complete a thorough write-up of what the amp sounds like. If you want to read the latest developments, check back in from time-to-time for news of additional tweaking, tone reports and clips. Now, I think I’ll go play some amp, er, I mean guitar :>)
My amp and me
15 – Playing the Amp
Here’s a clip of the amp. It’s the neck pickup of a Strat played through the Vibrato channel using some reverb.
Strat with the neck pickup through the Vibrato channel (MP3)
Day
008 - Well,
I played my first gig with the amp today and it played beautifully. It
had all of the brightness, punch, soul and grit that I needed for a full day of
music. I decided to play with nothing other than a guitar, a cord and the
amp.
It
handled a variety of songs. I'd change between the Normal and Vibrato
channels depending on the tune, adding a touch of tremolo here and there.
I also played around with the reverb settings using more or less, depending on
the song. On the tune, Secret Agent
Man, I maxed out the reverb and turned the tremolo up quite a bit and it
sounded like the Ventures playing surf music.
Really nice. I was able to get
down and dirty with the Normal channel and the Seth Lover humbucker in the
bridge position of the Hamer guitar.
During a
break, we took a little time to record a finger picking solo using the neck
pickup of a Fender Stratocaster. Give it a listen. There are plenty
more sounds available in this amp but this clip gives a nice demonstration of
the clean tones of the Vibrato channel. Note the increased reverb and a
splash of tremolo right at the end of the clip.
Day
001 - I played the amp for an hour when I got it home from Mikey’s
shop. Here are my first impressions.
In my music room, I could really tell I was going to love this thing. I plugged my guitar into my Klotz cable and then into the Vibrato channel. As I played complex chords with a touch of reverb, magnificent, three-dimensional sounds bloomed from the amp. I love the complexity of the tone. The note attack is crisp and bright but it morphs immediately into a smooth tone with a lot of sustain. With the guitar volume cranked up, this channel can give a stinging, bluesy tone that really turns me on. I turned the volume back down and added a touch of tremolo. It pulses beautifully, integrating into the overall sound.
On the Normal channel, there’s some more gain and more of a mid-range voicing. There’s not a hint of buzzy preamp distortion though. Great clean tones can also be extracted from this channel, but crank it up and man-oh-man. I particularly like the “woman tone” I can get with the Seth Lover humbucker in the bridge of my Hamer Artist Custom guitar. By turning the guitar tone knob all the way down, the amp does all the work of providing the treble. The lead sound is both smooth and aggressive at the same time and the tone sounds like a powerful, low moan without getting muddy at all. Just great.
One of the real pleasures of this amp is the bass response and the total lack of the speaker flubbing or farting out. Even at higher volumes and on both channels, the low E string can plink like a piano string. Wonderful. If a pedal steel player needed a low wattage amp, this would do the trick.
After I stopped playing, I went into the family room to say goodnight to my 17-year-old daughter, Alice, who was enjoying hanging out with some friends. The first thing she said was "Your amp sounds real good Daddy" (and she almost never comments on my playing!). Then she said, "I don't usually pay attention to your amps but tonight it caught my ear. It sounds real smooth."
I took this as a high compliment indeed to the quality of the amp sounds. Then I went upstairs and my wife said it sounds great and it is clear and clean without being too clean. She said it reminded her of hearing a high-quality bell. I guess the Reno family likes this amp!
16
- Contact Information
Wayne Reno |
Mike McGuirk |
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Lancaster County, Pennsylvania (USA) |
Baltimore, Maryland (USA) |
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I hope you enjoyed this article. If you have any questions or comments, or like what you have read, please feel free to drop me a note. You’ll also find me hanging out at some of the discussion groups I’ve mentioned in the article. I usually just use Wayne as my user name so I hope to see you online.
Wayne playing a 2001 Hamer Artist Custom guitar
with Crystal City at Cowboy Bob’s Music Festival
If you’d like to hear one of my original tunes, here’s a link:
This song tells the story of a fellow that goes to Jamaica to learn to play reggae music but they send him home because he always sounds like The Beach Boys singing Kokomo :>) It’s a fun song but it is also a lament because it is hard to make a go of playing reggae music in the USA if you aren’t from the Caribbean.
The band is called The Crick Rats.
On this tune, I play lead guitar, sing lead vocals and play the wah-wah rhythm track.
The deep, “reggae mon” bass voice is Rick Bowman, our drummer, who also produced this song at R&B Studios, which he shares with his wife Bev who plays bass on the track. Of interest, Rick is in the process of using R&B studios to produce a Johnny Cash Tribute CD. The other members of The Crick Rats band on the recording are Keith Van Zandt on the skank rhythm guitar and Kevin W. (he likes anonymity) on the Hammond organ. Everyone sings on the backup vocals.
Bev and Rick Bowman
Rick and Bev have an interesting business. Rick does fabulous custom and restoration work on motorcycles. Bev does custom leatherwork. You can see some of this fine work on their website:
I play in another band that is called Crystal City (an exploratory jam band). Our drummer’s name is Kurt Kauffman and I’ve been playing with him since 1985. Sometimes our friend Fred Amendola sits in with this band and The Crick Rats. He's a mean pedal steel player and a fine guitarist. My oldest musical buddy is our bass player, Steve Singer, who I have been playing with since 1975 in a variety of formats.
Crystal City
(Steve on bass, Kurt on drums, Wayne on guitar)
If you are in the area and need a band, we’ve got you covered!
Finally, I’d like to mention my beautiful wife, Sue, to whom I have been very happily married since 1975. In addition to raising our kids, we are great friends and we support each other in our artistic interests. Her patience with my guitar playing and tube amp hobby cannot be beat. In fact, she plays the Dobro a bit and we’ve made sweet music together over the years. Her artistic endeavors though are primarily focused on Fiber Art, which is the use of fiber for innovative wall hangings. Here’s an example of one of her pieces, which will be on display in a national show next month.
Sue’s fiber art piece titled “Poke Salad”
Over the years, Sue has developed quite the body of work that many people have found rewarding to have in their homes and offices. If you would like to see more of her pieces and keep informed of where her pieces will be displayed, you can check out her website at:
Thanks for stopping by my website. I hope it enlivened
your day a bit.
Peace,
Wayne
Reference Information:
As promised earlier, here’s the tube amp reference information I keep at my house. I don’t pretend to know even 20% of what is contained in these books and videos, but you have to start somewhere. I find that as I learn more, I go back to these sources when I am able to grasp more sophisticated concepts. Here’s the list:
1. “How To Service Your Own Tube Amp” (book and video) by Tom Mitchell – If you only buy one, this is it, in my humble opinion.
2. “Video 1 - Tube Amp Basics for the Guitarist” by Gerald Weber – I found Gerald’s videos and the one above by Tom Mitchell very, very helpful. It’s great to be able to see and hear about tube amp circuits instead of just reading about them.
3. “Video 2 - Basic Tube Guitar Amp Overhaul and Servicing” by Gerald Weber
4. “Video 3 - Advanced Tube Guitar Amp Overhaul and Servicing” by Gerald Weber
5. “A Desktop Reference Of Hip Vintage Guitar Amps” by Gerald Weber – Lots of good info and schematics
6. “Tube Amp Talk For The Guitarist And Tech” by Gerald Weber – more good info. Check out “The Trainwreck Pages”
7. “The Ultimate Tone – Volumes 1 and 2” by Kevin O’Connor
8. “The Tube Amp Book” by Aspen Pittman of Groove Tubes
9. “Tube Amp Workbook” by Dave Funk
10. “Audio Cyclopedia – First Edition” by Howard Tremaine – The bible, with everything knowable about tube circuits. Very technical. Make sure to get the First Edition since after this one a lot of the tube circuits were dropped in favor of solid-state. I bought mine used on eBay and it wasn’t cheap.
11. “RCA Receiving Tube Manual”
12. “General Electric Essential Characteristics”
13. “Tube Substitution Handbook” by Howard Sams
14. “The Fender Amp Book” by John Morrish – a small book with the Fender models and specs on each
15. “The Amp Book” by Donald Brosnac – an introductory treatment
16. “Amps! The Other Half Of Rock ‘N’ Roll” by Ritchie Fliegler – a good history of tube amps with lots of photos
17. “The Complete Guide To Guitar And Amp Maintenance” by Ritchie Fliegler – good practical advise but not very detailed
18. “Fender Amps. The First Fifty Years” by John Teagle and John Sprung – good history of all of Fender’s amps with photos
19. “The Fender Inside Story” by Forrest White – an insiders view of the history of Fender
20. “Marshall. The Illustrated Story Of The Sound Of Rock” by Michael Doyle
21. “Ampeg. The Story Behind The Sound” by Gregg Hopkins and Bill Moore
22. “Blue Book Of Guitar Amplifiers” by Zachary Fjestad – Specs on many brands of amps and what they are selling for now
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