If your question isn't answered in any of these FAQs, ask it on alt.guitar.amps !
There are a variety of locations on the web. We suggest starting with Duncan Munro's site at http://www.duncanamps.com/ . Duncan maintains a huge database, available on the web and as a free app you can download for use on a PC. He includes links to other sources as well.
Watts are watts. But a tube amp with a certain wattage rating will likely be capable of producing louder, more pleasing music than a solid state amp of equivalent wattage.
Watts RMS indicates how much a particular AC signal can generate, regardless of the shape of the wave.
If you look at any audio amp's output rating, it will indicate X watts at Y-frequency with no more than Z% distortion (probably across a certain frequency range).
Once you hit the limits of the amplifier, driving it past that point results in rapidly increasing distorion as the circuit begins to clip.
What is different about tube amps is the quality of the distortion is not as aesthetically displeasing as the quality of distortion in a transistor amp pushed to the brink. Also, in many tube circuits, the rate of distortion increase is not very steep, but gradual. In many solid-state circuits, the distortion is like a cliff, you reach a point and you cross the line and the circuits distortion increases dramatically.
Tube circuits tend to have a compressive effect as the maximum power level is reached. This tends to increase apparant loudness as well.
Another difference commonly found between tube and solid state amps is the application of global negative feedback. Many solid state amps have very high open-loop gain and rely on large amounts of negative feedback to linearize the circuit globally. Once clipping is reached, the stability of the feedback circuit is substantially compromised, again resulting in harsh, unpleasant sound.
Conversely, the typical tube amp uses relatively small amounts of negative feedback or perhaps none at all. The effect is similar and again contributes to a smoother transition into clipping, increasing appearant loudness.
So what you hear is an appearant loudness difference, rather than actual difference in electrical energy output. Loudness is a psychoacoustical phenomena, not a physical one. From the standpoint of physics, watts are watts. No more no less.
It is just like the difference between pitch and frequency. Frequency is a physical measurement that indicates a precise number of cycles per second.
Pitch is the perception of frequency. You can have exactly the same frequency at a low level and a high level and people will experience them as different pitches. The physical measurement hasn't changed, but the psychoacoustic experience of the frequency is different.
-Nuke
There are three basic functions for capacitors in tube amps. First, as other folks have mentioned, are the filter caps-- the big electrolytic "cans". As Robert points out, these are there to reduce the amount of hum from power supply ripple (not the wine). These caps do indeed "wear out" (dry up, develop leaks, develop weak spots in electrolyte if unused) and may need replacing. A good tech can check this and replace as needed. For very old amps, many folks just assume that they are all used up and replace every one of them.
The second function is a cathode bypass cap. I don't want to get too technical here, so let's just say that they help stabilize the bias on your preamp tubes (some amps also use them for the power tube bias). These are often smaller electrolytic caps, and can also wear out. If they're bad they often effect the overall gain, particularly in the low freqs.
Finally, there are coupling caps. These provide isolation between stages of the amp (or between the input and the first stage). They typically do not wear out, (except the older, paper/oil types) but replacing them will likely change the tone of your amp. The exact type of aural change is a matter of great debate in the high-end audio world with such words as detail, clarity, soundstage, imaging - you get the idea. You may want to play around with these, but the best way to do it is to replace them one at a time, and then play the amp to evaluate the effect. If it doesn't sound better, then put the old cap back (I know, I know, this technique may result in a local maxima that's sub-optimal, so shoot me). This approach takes some technical savvy (or a very patient tech), but it can help tweak the last bit of tone from an amp. (In general, larger values add bass and some mids, and may up overall volume a tad; smaller values do the opposite.)
Oh yeah, the fourth of the three types of caps are the tone filter caps - they're kind of like coupling caps in the way they effect tone. Changing the type of caps used here can change the "voicing" of the amp. Try it and see.
-Punkerdubh, with changes by Danny and Miles
Other uses are extensions of these. For instance, a parasitic oscillation suppressor may be a bypass or tone cap. The cap in the presence (negative feedback loop) circuitry is a type of tone cap.
-Miles
Here's how to bias it using the transformer shunt method. Looking at the amp from the front, just to the left of the power supply in front of the tubes is a white wire that plugs into the circuit board. Pull this wire from the board and clip one of your meter leads to the terminal on the board and one to the plate of a power tube. Add a jumper wire from the white wire you disconnected to the terminal on the circuit board that it was connected to. Turn the amp on and bias using the shunt method as you normally do.
-Tonefactor
12xxx tubes such as the 12AX7, 12AT7 and 12DH7 are really 6V or 12V tubes, depending on how they are wired. The heaters are wired like this inside the tube:
4 -------
|
+----- 9
|
5 --------
If you hook 12 volts across pins 4 and 5, with nothing across
pin 9, the heaters are in series, and run at 12 volts. If you
tie pins 4 and 5 together, and run 6 volts across that junction
and pin 9, the heaters are in parallel, running off 6 volts.
Each leg of the heater is set up for 6 volts, so you can run
them in series or in parallel. Kind of like speakers and the
impedance (ohms) questions.
-Miles
It can't. Typically, if you have 6 and 12 volt tubes mixed in one circuit, everything runs at 6 volts (see previous question). You could also have a 12V heater circuit with a center tap, so that 6V tubes can be run off each half of the 12V winding, between one of the outer legs and the center taps (this was common in tube-based TVs). Finally, you can string two 6V tubes' heaters in series to run them off 12V.
-Miles
From the ARRL handbook:
BLA - grounds, grounded elements, returns BRO - heaters/filaments, off ground RED - power supply B+ ORA - screen grids (and base 2 of transistors) YEL - cathodes (and transistor emitters) GRN - control grids, diode plates (and base 1 of transistors) BLU - plates (and transistor collectors) VIO - power supply, minus leads GRA - AC power line leads WHI - bias supply, B or C minus, AGC
-Miles
From Orr's Radio Handbook:
Primary leads -------------- black
(if tapped)
Common ----------------- black
Tap -------------------- black/yellow
End -------------------- black/red
High voltage secondary ----- red
Center tap ------------- red/yellow
Rectifier filament winding - yellow
Center tap ------------- yellow/blue
Filament winding No. 1 ----- green
Center tap ------------- green/yellow
Filament winding No. 2 ----- brown
Center tap ------------- brown/yellow
Filament winding No. 3 ----- slate
Center tap ------------- slate/yellow
I have seen primaries with other color schemes;
if it includes black it should be a
primary lead of some sort. I've seen transformers
with only one filament winding that was brown.
I've seen high voltage secondaries with red/white
centers. And on old transformers, the colors may
have faded to the point brown looks black or green
looks black or brown, and so forth. I'm not aware
of any standard for filament winding numbering, so
if there is more than one, verify the voltages.
The rectifier winding will normally be 5 volts,
but again, verify it. If you have an old tranny
with odd or indistinguishable colors on the leads,
verify them. In fact, it's a good idea to verify
any PT before using, just to be safe. Miswirings
are, AFAIK, rare, but do occur, as do shorts.
-Miles
Derived from the ARRL Radio Amateur's Handbook:
Single-ended transformers:
Plate lead (pri.) -------------- blue (or brown) B+ (power supply) lead --------- red speaker (typ. +) (sec.) -------- green (or yellow) speaker return (sec.) ---------- blackPush-pull transformers:
Plate lead (pri.) -------------- blue (start) B+ (power supply) lead --------- red (center tap) Plate lead (pri.) -------------- brown (finish) speaker (typ. +) (sec.) -------- green (or yellow) speaker return (sec.) ---------- blackOrdinarily the black side should also be grounded, if the speaker is grounded. Replacement transformers are not always identical, so if you get squeals or other odd sounds when hooking up a new output transformer, try reversing the output leads. "Start" and "finish" are arbitrary terms with respect to this configuration.
-Miles
From the ARRL Radio Amateur's Handbook:
Plate finish lead (pri.) ------- blue B+ (power supply) lead --------- red (whether center tap or not) Plate start lead (pri.) -------- brown Grid finish (sec.) ------------- green speaker return (sec.) ---------- black (whether center tap or not) Grid start (sec.) -------------- yellow
-Miles
From the ARRL Radio Amateur's Handbook:
COPPER WIRE TABLE
Ohms/ Ohms/
AWG[1] Amps[2] SWG[3] foot[4] meter[4]
8 46.0 10 0.00064 0.00022
10 33.0 12 0.00102 0.00036
12 23.0 14 0.00162 0.00057
14 17.0 16 0.00258 0.00090
16 13.0 18 0.00409 0.00172
18 10.0 19 0.00651 0.00228
20 7.5 21 0.01035 0.00362
22 5.0 23 0.01646 0.00756
24 3.5[5] 25 0.02617 0.00915
26 2.2[5] 27 0.04162 0.01457
Notes:
Instructions are available at http://www.rru.com/~meo/Guitar/Amps/PA2Guitar/old2new.html .
* Lower impedance will _generally_ stress the power tubes gradually more and more as the volume goes up.
* Higher impedance will _generally_ stress the power tubes less as the volume goes up until the powertubes cut off at high volume levels. This will create voltage spikes on the plates of the power tubes called flyback.
If the flyback spikes are kept within sensible limits (say 1000V-1200V for EL84/6V6 etc., and 1500V for 6L6 type amps), then it's unlikely that any harm will be done. It's when they go way higher than this that the tubes, transformer or sockets can flash over causing expensive damage to the amplifier.
In summary, lower impedances _may_ stress your power tubes and shorten their life. Higher impedances are OK to a point, then it becomes Russian Roullete.
Where running high impedances are concerned, much depends on:
* Quality of output tubes * Material used for the sockets * Leakage inductance of the output transformer * Quality of insulation in the output transformer * Amount of negative feedback in the power stage * The volume you play at * The level of mismatch
For running low impedances, the amount of extra stress will be influenced by:
* The class of the amplifier (A/AB) * The volume you play at * The level of mismatch
Given all of the variables above, it's difficult to hand out blanket statements like X is good but Y is bad. The only totally safe advice, is to ensure that what's stamped on the cab matches what's stamped on the amp.
If you can't do this for practical or emergency reasons, follow LV's advice and go low instead of high - there's a lot less to go wrong that way...
-Duncan Munro
The crux of the problem lies in the inductive nature of the output tranny. Inductive loads are pretty special things, since they STORE energy in a magnetic field. A property of this effect, as has been pointed out, is that the voltage can soar to levels above the supply voltage in the amplifier-- sometimes WAY above. You can't do that with any other kind of load other than inductive.
Now the transformer doesn't have an impedance of its own; it only reflects an impedance from one winding to another in proportion to the turns (or voltage-- they are the same) ratio squared.
So imagine that you've got an open secondary. This impedance is for all intents and purposes infinite. Thus, regardless of the turns ratio, the primary impedance is infinite as well. (leakage inductance and parasitic capacitance-- two unavoidable nasties of real-world trannies-- will limit this to some finite number less than infinity, but suffice it to say its really high.) This means that the primary will act like a constant current source, attempting to keep changes in currents through its windings to a minimum. This will be an important point later.
Operating into such a humungous load impedance will cause the plate to swing HUGE voltages, according to V=IR. Especially with tetrodes/pentodes, which are much better at cranking out current, the delta Ip will stay the same regardless of the load. Consider what happens when the R goes sky high.
Now, if the load were NOT inductive, the maximum possible voltage generated would be equal to the rail voltage. No problem. This is how it is in SS amps. But with tube amps, that's not the case.
The primary danger here is in the development of these extraordinarily high voltages, which can punch through winding insulation, arc over tube sockets, even arc inside the tubes themselves. Once an arc has struck you can be pretty sure it will happen again. And again.
This is not good. Probably the worst scenario is that the OPT primary arcs to the core, which is grounded, and that will cause mega current to flow. The OPT is toast, and the power supply will be too unless something stops that current in a big hurry.
So that's what can happen with too high a load. Admittedly, this is an extreme case scenario here, where you've got an OPEN secondary, and thus a very very high primary impedance to work into.
Notice above how I pointed out that tetrodes/pentodes will have a worse time of this than triodes. This is because of their much higher dynamic plate impedance, which can also be described as their being an approximation of current sources. The pentode will just keep cranking out plate current-- regardless of what potential the plate is. The electrostatic shielding effect of the screen grid continues to "pull" electrons from the cathode with the same force. Thus the plate current is largely independent of the plate voltage, a mark of high plate impedance. That current is what "builds up" (so to speak) when working into a high load impedance and generates the excessive voltages. It's almost as if you've got a constant current load (the unloaded OPT) on a constant current generator (the plate of the pentode)-- obviously if these two devices are "concerned" with currents, not giving a flip about the voltages involved, you can get some pretty crazy effects.
With a triode, the much lower plate impedance limits the extent to which the plate voltage will swing about uncontrolled. As the plate swings high, for example, the attraction of electrons from cathode to plate will increase due to the higher voltage. More electrons will be pulled to the plate, regardless of what the control grid is doing. More negatively charged electrons means less positive voltage, so the voltage is "automatically" decreased. This is a direct measure of plate impedance.
In fact, running a triode into a very high impedance is done all the time with interstage transformers, which generally are very lightly loaded. The inherent degeneration in the plate circuit keeps the peak voltages from becoming a problem. Actually, triodes "love" current loads of very high impedance-- the tube is operated in its most linear fashion and is free to do what it does best-- generate an output VOLTAGE.
You can think of the dynamic plate impedance of the tube as forming a voltage divider, with the inductive tranny between plate and B+, and the tube itself between plate and ground. Obviously, with the low plate impedance of a triode, the voltage cannot swing madly about. Now consider the very high plate impedance of a pentode, and how much higher those plate voltages can swing.
OK, that's the situation of too HIGH a load impedance. So what about too LOW of an impedance? Let's consider a dead shorted secondary on the OPT.
Now the primary presents a very low load to the tube, a low impedance, a vertical load-line. We will notice that the tables have exactly turned.
Since the triode's plate is like a voltage source, it will attempt to pass incredible amounts of current in a heroic attempt to make the plate voltage swing. Operating into a dead short, it cannot do this, so something eventually will give. The cathode will attempt to emit way more electrons than it can, and it will have a short, hot life.
The pentode, however, is more of a current source, so it will continue to pass the total plate current in accordance with the screen voltage and the control grid voltage. These have not changed with the alteration of the load, so the pentode will continue to merrily pump its current swings into a dead shorted load.
Take a look at some plate curves, if you need to. Find some for pentodes and for triodes. Better yet, find some for the same power pentode connected as a triode (g2 connected to anode).
First the pentode case: look at the way the curves lie on the page. Imagine a horizontal load line (infinite load, open secondary) drawn on the graph. Notice how the pentode doesn't look like it would work this way-- an infinitesimal control grid voltage change would produce a gargantuan change in plate voltage. The tube is NOT happy. Now imagine a vertical load line (zero load, shorted secondary). The pentode's peak current for a given control grid voltage doesn't change much at all-- the vg1=0 plate curve is nearly horizontal for most power pentodes, cutting right across all of the various plate voltage points. It doesn't matter what Vp is at all-- no matter where you draw that vertical line, the peak plate current is pretty much the same. The tube is happy.
Now the triode case: imagine the horizontal load line now. Notice how the plate voltage is almost PERFECTLY proportional with respect to control grid voltage. No matter which tube you try, or what current you draw that horizontal line at, it will be VERY linear. The tube is happy. Then consider the shorted output tranny case, with a vertical load line. Notice how the vg1=0 curve will produce a humungous plate current since the plate curves are so much "steeper" than the pentode's case. The tube is NOT happy.
What the heck does all this mean? Well, hopefully you aren't running ANY tube amp into a shorted or open load... Since no pentode is a perfect current source, and no triode is a perfect voltage source, the actual characteristics are somewhere between the two idealized cases. As LV pointed out, you're much better off running a pentode amp into a lower load impedance than it expects. For those of you with triode output tubes, or a triode switch, you're better off running into a HIGHER load impedance. If you don't see why by now, reread this essay. It's also a good idea to take a high value power resistor, say 470R, and hard wire it right from the OPT secondary's 16R tap to ground. This will dissipate a very small amount of power under normal conditions, but will limit the extent to which the primary impedance will tend towards infinity in the case of a disconnected load.
For what it's worth, I've been deliberately "mismatching" load impedances by one tap for years. In other words, either a 4R or a 16R load on an 8R tap, and so on. This small mismatch will limit output power and will change the clipping points of the output tubes, but will not damage anything in a properly designed amplifier. Keep in mind that a higher load impedance in a pentode amp will put additional stress on the screens, so you may want to have at least 1k stoppers installed. A lower load impedance will cause more plate current to flow, and if you're running the tubes at the edge of acceptable quiescent plate dissipation that may push them over the edge into the red zone. If you've got an old vintage amp you'd hate to see get damaged, by all means, play it safe and don't mismatch at all. But if you're wondering about how it sounds, and the amp's got good trannies in it, then mismatch away. Just keep it within ONE TAP please, for safety's sake.
-Ken Gilbert, by way of PMG
I'm going to discuss feedback in general first, in terms anyone who's ever been around a sound system or amp can understand, then move into the details of your question. The early bits may seem obvious, but should help you understand the details farther on. Plus, I'm trying to make this general enough that anyone who's reading this has a good chance of understanding it.
In general, feedback is when you take an output signal, and feed all or part of it back into an earlier stage. This is the same principle whether you're talking amp design, Hendrix feeding back on stage, or unwanted mic howling.
Positive feedback is when the output signal is in phase with the input signal, so that it reinforces it. You take a small signal, amplify it to a larger signal, then feed that larger signal back into the amp, so that it comes back even larger, and so on. Eventually, if you don't tame that signal, something gets to oscillating and you get a sustained feedback howl (or screech or whatever). This is the kind of feedback sound guys hate, and rock guitarists love when they can manage it.
Some sound boards have a phase switch on the mic inputs. (You can also buy or make boxes with phase switches in them to put inline in for the patricular mic you have a problem with.) These swap the signal lines, so that now the feedback is out of phase, and cancels itself.
Negative feedback (also called inverse feedback or degenerative feedback) involves feeding the output signal back out of phase with the input signal, which can have a dampening effect. (Just like the phase siwtch example above.)
You can build electronic feedback into a circuit. In this case you don't have an acoustic component (speaker and mic or guitar pickup); you just feed the electrical signal back to an earlier stage of amplification. The results are similar, except that with positive feedback instead of a howl, you may simply max out the amp, or fry components.
Op amp circuits use negative feedback all the time as a means of gain control. Regular transistor circuits do this far less, and tube circuits even less. With one exception...
The negative feedback you specifically seem to be asking about is usually a case of taking the signal from the output transformer's secondary (the speaker side) coil and feeding it back (through a resistor to drop the amplitude) to a point earlier in the circuit, where that output signal is 180 degrees out of phase with the input. It's typically injected into at the cathode of a phase inverter or late preamp stage. If a presence control is present, it is connected at this same point, and uses a pot and cap in series to ground, thus shunting some amount of high frequency signal to ground. Conversely it can be seen as a variable cathode bypass cap that just happens to be used along with the NFB.
The NFB loop sometimes runs into the tone circuit instead of into a cathode of a PI or preamp tube. On PIs that require two triodes, such as a Fender long-tailed PI, the signal is also injected into the inverted side's grid, via a cap, as well as into the cathode.
Negative feedback is typically used in audio amps to reduce distortion. It can also help keep overall circuit gain constant even with line voltage fluctuations, or caused by differences in tubes or variations in other component values.
An amp properly designed with NFB may not do so well without it. Removing the NFB may cause the amp to oscillate or go into thermal runaway. OTOH, if NFB wasn't a critical component of the amp design, it may not hurt to remove it, and some amps reputedly sound better without it.
I do not know whether any guitar tube amps were actually designed to require NFB for proper operation. If you are thinking of removing the NFB loop from your amp, check with someone who knows that amp well, or at least be ready to monitor it closely at first when you remove the NFB.
NOTE: Since the signal is taken from the OT, it can be injected back either an even or odd number of stages, just by switching which side of the OT the tap comes from. Obviously, the opposite secondary leg will then be the one grounded (if any).
-Miles
Yes. If you're worried about the amp's value to the insane collectors who even worry about whether the dust is original, keep the old cord and the "death cap" if any. But do the work or have it done. This can save your (or someone else's) life.
If you're good with a soldering iron and are aware of the safety issues (poking around in an amp can hurt or kill you!), it's one of the easier mods to perform. But it must be done right, or you could be closer to death than you were before. You can read up on the safety issues and the conversion itself at http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/safe.html .
There are two causes of speaker failure - electrical and mechanical. Electric damage is usually due to excessive heat in the voice coil. This is why a 10 watt speaker can last for a long time when driven by a 50 watt amp under certain conditions (i.e., time to cool off as occurs with very clean sine wave signals). Mechanical failure occurs when the suspension is compromised by the stroke of the piston action (excursion) exceeding design limits. This condition is interactive with electrical as it takes a given quantity of power and frequency to cause a speaker to jump that far. Point is it is not as simple as matching "rated output" with "rated handling".
First of all it is very important to realize that wattage is not static. The output of your amp is not a steady tone and when it is driven into clipping at some point the rise time is so fast it is difficult to discern it from a square wave, representing many attributes of a much larger output. Simply put, wattage is at best a guideline and by itself is insufficient to describe what is happening in this case that is capable of blowing a speaker.
A speaker's impedance changes with frequency and its excursion (the stroke of the piston action) changes with both frequency (much greater with bass notes) and volume levels. An open back cabinet allows unrestricted excursion compared to a closed box. Additionally if the speaker is rear mounted (the front of the speaker is flush with the back or inside of the baffle) it is possible to skew the voice coil off axis by unmatched tightening of mounting bolts causing early failure. Front mounting is not failsafe but is generally less strenuous on the speaker basket depending on whether the basket is cast (strong) or stamped (weaker) and how thick and spongy the gasket material is.
If all this seems confusing, the best thing to do is to examine one or more blown speakers to determine what manner of failure is occurring. Is the surround or the spider ripped? If not use a razor to cut out the surround and the spider and remove the cone and voice coil as a unit. If there is evidence that the coil of wire has rubbed (you will see the clear enamel-like insulator has been scratched) then overtightening is likely. Both of the above are mainly mechanical failure.
If the coil is unscathed but is melted like a fuse then the problem is electrical failure. In a solid state amp this can be caused by DC voltages leaking from outputs. In tube amps it is from being overdriven electrically, usually from excessive clipping, impedance mismatch, etc.
The solution, assuming you must play at these levels of volume, frequency, and overdrive is to limit the mechanical or electrical cause. In some cases closing or porting an open back cabinet can solve the problem. Addding a second speaker enclosure is a great solution as it adds electrical handling and halves the excursion for a given frequency/volume assuming the speakers and enclosures are at least similar. Otherwise the only choice is to choose a speaker which has much greater power handling capacity.
This presents an additional problem because unless it is also as efficient as the lower powered speaker it will not be as loud at a given amp output. For example a speaker rated at 100db/1 watt/1 meter will be 1/2 as loud as a speaker rated at 103db/1watt/1meter. So unless you are acquainted with a speaker that handles a lot more power just as efficiently, and has the tone color you like, the best solution is multiples of the one you already like. If you play a really wide variety of venue sizes either build or buy a number of single speaker enclosures (modular), dual speaker cabs, or go the tried and true 4 x 12 route.
Remember, an amp doesn't know how many speakers it is connected to. All it sees is impedance. A 30 watt amp will happily drive say 100 speakers if the total system impedance is matched to the amps output requirements (or theoretically infinite speakers as long as Z matches). In fact it will be more efficient because each magnet and voice coil is effectively an electromechanical engine.... more engines, more efficiency.
-Jimmy
These refer to how an amplifier stage is biased. With guitar amps, the only place they mean much is in the output stage, since pretty much everything up to that point is always Class A. Note that the following definitions apply to how the amp is biased to run at maxiumum clean volume without exceeding the tubes' rated maxiumum power dissipation.
All single ended amplifiers (whether they use one output tube, or more than one in parallel) are biased Class A, unless they distort. (More on this below.) I'm not aware of any guitar amps biased Class B (much less C, D, H or any other class). Most guitar amps with push-pull output stages are Class AB.
``The suffix 1 may be applied to the letter or letters of the class identification to denote that grid current does not flow during any part of the input cycle. The suffix 2 may be used to denote that grid current flows during part of the cycle.''
-RC-22 again
``The uninformed teeming millions assume that the letters applied to different amp classes are some kind of grading system that indicates what the sonic goodness is, which of course is not the case at all (except that Class C is pretty useless for audio except as a distortion generator).''
-"Uncle" Ned Carlson (thanks to Nick Dolling for catching this)
Guitar amps can, for practical purposes, be biased as Class A, although technically, if the waveform can be driven into distortion where the negative peak is clipped at cutoff, meaning no plate current flows, then the amp is not Class A. A [perhaps] less technically correct, but more useful approach, is to consider that if any given tube is driven into clipping and cutoff at the same level, it's still biased for Class A. Yes, this may violate the "letter of the law". But it's a useful concept. And, in fact, the RCA manuals refer to things this way.
On the other hand, to speak of an amp that is biased for Class AB but operating in Class A, or vice versa, is just plain nonsense.
-Miles
What does all this mean, in practical terms?
For a detailed explanation, check Randall Aiken's excellent presentation at http://www.aikenamps.com/, selecting "Tech Info", then "The Last Word on Class 'A'".
Sure, lots of thoughts... and not all of them are from the voices, many are my own. ;-)
I have a lot more hand-on (intentional typo) experience repairing pre-WW2 radio gear, but have formed some general ideas regarding miscellaneous popping sounds in tube amps (really any tube gear). The causes probably go like this in order of commonality:
-Zenon Holtz
(The following posts include some overlapping information, but reading them all will give you the full picture. -ed)
6L6 is the metal version, made from beginning to end.
6L6G is the large glass version, ST-16 envelope, made from the 30s into the 60's.
6L6GA is an ST-14 version from around WW2, same ratings as the 6L6G, just smaller glass envelope. I like them as they look right with 5V4G rectifiers.
6L6GB is the T-14 version of the 6L6G and GA. Size is the same as the GC version except the published ratings are the same as the GA and G versions. 6L6GB has the crimp seal where the element mount is like earlier production glass tubes, and GC has a glass seal at each pin like a big compactron with a bakelite shell base on it. Seem to be from 1958-63 (RCA)
6L6GC has increased voltage and power dissipation ratings compared to the other glass and metal 6L6 versions and uses the new construction element mount. This construction lets more heat escape through the pins, I will remember it's proper name just after sending this off, I'm sure. You are still better off staying within the other versions' voltage ratings in my opinion with the GC version. If you are going to run 450 volts get the 7581A/KT66.
5881 is a special construction 6L6GB electrical equivalent. Unsure of the original design purpose myself. They are the smallest of the 6L6 variants. Likely the best choice for an oscillator circuit.
-Jeff Goldsmith
6L6, 6L6G, 6L6GA, and 6L6GB have identical ratings (max plate 19 watts) and differ only in bulb size. The 6L6 has a metal, MT-10 sized bulb. The 6L6G has a glass ST-16 bulb. The 6L6GA has a glass ST-14 bulb and the 6L6GB has a glass T-12 bulb.
Note: MT = metal tubular, ST = shouldered (spherical tubular), T = Tubular. The size is the widest dimension in eights of an inch. Suffixes such as "Y" (eg 6L6GAY) and "X" (eg 6L6GX) represent tubes having low loss (higher insulating) bases. "X" means ceramic based, and "Y" tubes are brown based, the usual material being micanol. These "X" and "Y" suffixed tubes were usually designed for transmitter use which required higher frequency capability.
The 6L6GC has the same bulb as the 6L6GB but is uprated to 30 watts max plate.
The 5881 (aka 6L6WGB) is mechanically ruggedized. It has a T-11 bulb and is rated at 23 watts mx plate.
The 5932 (aka 6L6WGA) is mechanically ruggedized. It is rated at 21 watts max plate. It has a T-12 bulb, but the base is a bit bigger and may not fit in tight spaces designed for metal 6L6s or 5881s.
The 1622 is a low noise metal 6L6 (MT-10) derated to 14 watts.
The 1614 is a transmitter-rated (up to 80MHz) metal (MT-10) 6L6 with a max plate of 21 watts.
The 1631 is the 12 volt equivalent of the 1614.
-Jim Cross
The 6L6EH is the current Electro-Harmonix 6L6GC variant, and is not just a specially selected or rebranded Sovtek 6L6.
-Miles
Changes in manufacturing techniques is likely the main reason for the different envelopes in the 6L6 G, GA, GB/GC. The 5U4 went through a similar transformation over the same years and the type 80 went through several generations of change from globe to button base T-9. Back compatiblility was generally preserved with the changes.
The early change from almost self supporting elements in globe envelopes to mica top element supports in tubes like 201A were to gain more uniform characteristics from one to another (Cunningham TM - 1927), but the structure could bend over easily without a support at the top to hold it upright, and the ST envelope was introduced with the mica top support and all earlier types in globe envelopes were slowly changed over to the new design. 46, 50, and 866 were available quite some time into the thirties in the globe envelope.
Likely the 5881 was specified in the beginning by Uncle Sam. At the time it used construction that was not used in 6L6 tubes yet. The ratings were acheived by using GC element mount construction but only going for GB ratings so it can be smaller then the other 6L6 variants.
-Jeff Goldsmith
You can find more information on 5881s and 6L6s at http://www.jt30.com/jt30page/micKtubes/What-is-5881.html. <
[Thanks to dr wow for that link!]
Not much really. A 6L6G might not fit in equipment designed for a 6L6GB. Theoretically, a 6L6G would dissipate heat a bit better due to its larger glass area.
-Jim Cross
Push-pull amps have the inherent characteristic of even order harmonic distortion cancellation. The more carefully the amp is AC balanced, the lower the even order products. This means a couple options exist:
I've watched on my spectrum analyzer as I adjusted the AC balance on an amp. What occurs is that if you "unbalance" the drive to the output tubes in the "right" direction (you need to be able to see on the analyzer to know which way is "right") the 3rd, 5th, 7th, etc. odd-order harmonics increase VERY slightly, but the 2nd, 4th, 6th , etc. even order harmonics increase significantly. The second increases the most.
Even in audio gear, the optimum distortion pattern seems to be a gradually descending amount as the order of the distortion increases. The distortion products decrease at a fairly constant rate as the order increases.
(Note: order simply means the multiple of the fundamental frequency. 2nd harmonic is 2x the fundamental tone, 3rd is 3x the fundamental, 8th is 8x the fundamental, etc.)
In hi-fi gear, even though low distortion is desirable, the types of distortion often dictate the sound quality. The pattern above is desirable, with as few higher order distortion components as possible (odd or even order). BTW, SS audio amps tend to produce odd order distortion, tube audio amps produce more even order distortion, thus the common preference for the more musical tube amps. Even order distortion is considered more musical, more pleasing to the ear than odd order distortion.
It would appear that in instrument amps almost the same thing holds true. While low distortion is NOT a design goal of most instrument amps, clearly a musical distortion pattern could be. Maybe that's why a Champ has such a good tone (in many players opinion anyway) - a single ended amp has no cancellation at all! You get all the distortion products rather than an exaggerated proportion of odd order that occurs in push-pull amps.
LV or somebody could comment on this, but it seems to me you could try/hear this for yourself by making your push-pull amp into an SE amp (temporarily). You could lift the coupling cap to one output tube (of a push-pull pair). Leave the DC flowing to avoid transformer problems, and you could have a single ended amp to try. With one coupling cap disconnected you only get single ended AC. There are some limitations to this, such as a big loss of power/volume, but it seems like something I'd try if I was a musician seeking a different tone. I know, the NFB loop will try to compensate, and when I drive the output past its class A output level to AB the distortion will be VERY large since I'll be cutting off a big part of the waveform, etc. Might be fun to try though. Anyway...
Thanks to Randall for his really nice work (as always!!).
Jim McShane
The 90 degrees usually works fine but it can be improved. If you look at the transformer mounts in 80's home cassette decks you will see that there is actually a circular set off transformer mounting holes. During final testing they actually would set the transformer angle to minimize hum in the electronics (I believe the hum was mostly picked up by the playback head). I have seen these at various angles in the same model and production run (mostly in the higher end stuff tho'). Fender amps and many others used the Z mount for the power transformer (where the transformer is half way thru the chassis) and standard chassis mount for the power and choke to reduce hum induction. I believe potted transformers were an effort in this area as well.
-Matt Seniff
As far as you can get away with, without otherwise compromising the lead dress. If you aren't sure, experiment. If they're touching they're too close. On separate planets is probably not worth the hassle or expense.
-Miles
Microphony is caused by instability or play in the internal structures of the tube. Since the various elements inside a tube have various voltages applied to them, and since (like any two pieces of metal which are in close proximity) there are various inter-element capacitances involved, when mechanical excitation is applied to the elements these capacitances change, causing a signal to be developed on the tube's plate when, as a consequence of the changing capacitances, the voltages change. The more play there is in the internal structure, the more pronounced this effect will be. It is exaggerated in smaller tubes by the fact that the elements are in closer proximity and therefore smaller motions will cause larger signals to be developed. (This is why it's more common to find microphonic preamp tubes than microphonic power tubes.)
HOWEVER - and there is always a however - some of the best-sounding preamp tubes out there are microphonic. Microphony actually plays a positive role (when not excessive) in a preamp tube's characteristic sonic signature, and many of the most revered ones (Telefunken, Mullard, Ei, etc.) are quite microphonic. Microphony, in this case, provides a form of positive feedback which is often beneficial to the overall tone of the amplifier.
If you are rejecting tubes because they say "pow" or "clink" or "boing" or "wham" when you smack them with something, you are barking up the wrong tree. You haven't located a bad tube when you do this, all you've done is discover that a tube will make a weird noise when you do something weird to it. (Do you whack your tubes while you're playing the guitar?) A lot of perfectly good tubes have been tossed out by people who didn't know any better. As long as a tube doesn't induce any undesirable artifacts into the signal, like ringing, howling, rattling, etc., it's not too microphonic to use - no matter how loud it hollers when you spank it.
-Lord Valve
Bill Kahle:
In simple terms, this resistor is used for current limiting to prevent over dissipation of the g2 screen grid.Ken (ride5000):
bill's right. they also serve the secondary purpose of lowering the Q of the screen grid. in plain english, they make the screen much less likely to oscillate. this can easily happen if you have multiple screen grids fed from the same supply node without the stoppers.
Bill Kahle:
Not really.... It depends on the condition of the tube failure. Sometimes a failure will occur that involves the screen. If a short occurs that involves the screen and another [tube] element that is near or below ground potential, the excessive screen current will burn the screen resistor. This will shut the tube off.Ken (ride5000):Maybe others can elaborate on failure modes that involve the screen?
well, the simple fact is most (i'd say a good 90%) of tube failures are failures of the screen grid.. it's delicate yet it gets a lot of abuse and dissipates a fair amount of power, ESPECIALLY as the amp is cranked up and the plate voltage swings really start cooking. by protecting the screen with the stoppers, you are decreasing the abuse the screen gets and thus increasing the longevity of the tube. if the tube doesn't die, the OPT is not likely to either. so indirectly, yes, it does help the chances that the OPT survives.the downside is that they waste power (generating heat under the chassis) and they decrease the max Ip possible, decreasing ultimate power output. additionally, stoppers that are too high in value make the entire output stage seem spongy, since you are effectively increasing the plate impedance of the output pentodes/beam tetrodes.
True-RMS will give you the actual RMS voltage of a complex waveform. Other meters only really measure average voltage of an assumed sinusoidal waveform and then multiply it by a scalar to give you *apparent* RMS. Most meters (True-RMS and otherwise) have an upper frequency limit to AC voltage (often only a couple hundred Hz).
A good DMM will have a really high input impedance (40M or more), quick response and high voltage isolation. 1000 VDC and 700 VAC is what you should look for. Number of digits in the display isn't really too important for amp work. The more digits, the longer the response. Most meters will include diode checkers. Some have transistor checkers (although really, the diode checker works too). An audible continuity checker (beeps when sees a closed circuit) is really handy. You'll want a mA scale for checking bias by the shunt method. A bar graph display is handy sometimes.
Although it's hard to find in the sales brochure, better meters have lots of internal protection...you don't want to fry the meter if it's accidentally in the resistance function and you're trying to measure voltage!
Many have capacitance functions, but some have a limited range (many expensive flukes only go up to 5uF...not the 187, though). A DMM may measure capacitance, but won't measure cap leakage at voltage...a more common failure.
What's your budget? My current favorite (no pun intended) is a Fluke 187. True-RMS up to the high end of the audio spectrum, and there's a VERY cool dual-display function which will show simultaneously AC and DC voltage. Great for troubleshooting power supply ripple, and also for showing plate voltage *and* signal at tube anodes! NICE meter but costs around $300.
-Mike Schway
[The following were responses to a more sopecific question regarding why a 60 watt (2x6L6GC, rated at 30W each) amp should be biased at 22W per tube. -Miles]
The tubes Pd (plate dissipation) ratings are *maximums*. They are not centers, but maximum conditions that the tube is designed to withstand. If you run a tube at maximum dissipation, it will not last for very long, even if it is a really good NOS tube.
LV's numbers [22W/tube] are good rule of thumb operating idle conditions for push-pull class AB amplifiers. For class AB, we want to have some current flowing through both sides of the cycle until the amplifier reaches a near maximum output peak. This pushes the crossover or notch distortion out to the levels that we don't hear it.
As a rule of thumb, 75% of rated *amplifier* output is a reasonable *maximum* level you'd ever want. Every time you increase the idle current, the shorter the life of the tube. So ideally, you want to set the current as low as possible that is consistent with the tone you like. More idle current beyond the threshhold of good tone is just a waste of power and a waste of tube life.
So if you take your Vibroking's 60 watts (which it really can do, as per the specs Fender publishes, which is probably 1khz at 10% thd into a rated pure resistive load), divide by two output devices and multiply by 75% you get (60/2 * .75)= 22.5 watts, maximum, which is what he wrote.
If you find a bias level you like that's lower than that and sounds good to you, that's bonus money in your pocket! (usually, you will). In amps running at the common paramaters that a VK does, I find that 35 ma of plate current is usually satisfactory.
If you want more information on the topic of biasing and tube theory and operation, I'll refer you to Randall Aiken's excellent information website. http://www.aikenamps.com/ and look at the Tech Info link.
-Dr. Nuketopia
Technically, the 6L6GC is rated for 30 watts plate dissipation.
(RCA Receiving Tube Manual #RC-30, 1975 edition, page 353.) This is listed as a "design maximum" specification. It is common practice to run these tubes in class AB output configurations at 70-80% of the design max. (In class A designs, they are often run at design max - and beyond. Like Emeril says, "Kick it up a notch!) 70% of 30 is 21, 80% is 24. I pick 22-23 watts as a suggested *max* static dissipation wattage, since I don't know what kind of circuit my customer is going to put the tube into, and since I must assume he can't tell a phase inverter from a fire hydrant anyway. (No offense... ;-) (In fact, some of 'em can't tell a B-flat from a beer can either, but that's another story... ;-)
... [22 watts] is on the conservative side, but not "very" conservative.
If you look up the specs on the 7581A, you'll find that it's rated at 35 watts max. Now, if you were to disassemble a Philips 6L6GC and a Philips JAN 7581A and put the pieces side by side, you wouldn't be able to tell the difference - because there ain't any. The increased rating is just salesmanship. The fact is, these tubes are so well built that they can run at plate dissipation levels that would turn a Russian tube into slag in half an hour. That doesn't mean, however, that you *should* run them like that, because the hotter you run them, the shorter the service lifetime will be. Something you need to take into account is that they will pull *more* current (in a class AB output stage) when they are amplifying a signal than when they are just sitting there pretending they're space heaters. If you have them biased on the edge of red-plate at idle, they're going to go red when the rock 'n' roll starts...and you won't be watchin' 'em then, dood, you'll be staring at those [gals] bouncing around on the dance floor. Smoke time.
> In fact, Fender advertises the Vibroking (which is designed around the > 6L6GC) as a 60 watt amp. Unless there is special super secret Zinky > magic, that would be 30watts per tube ;-)I see this time after time. It's a mistake frequently made by noobs.
Pay attention, class...I don't want to have to repeat this (again). The static dissipation wattage is a measure of the *heat* generated by the tube at idle, NOT THE AUDIO OUTPUT POWER. That's it. It's only vaguely related to the audio output power, and not in the way you may think. Several times a month (more, if biz is good) I get a phone call from someone complaining that his tubes are defective (EL34s especially). I ask him did he read my biasing article. He says yes. I ask him what static dissipation wattage he set the tubes for, and he tells me 25 watts, because his 100 watt Marshall has four power tubes in it, and four times 25 equals 100. So, the tubes are turning red and they must be defective. Egad. I tell him to read it three more times, and if he still has questions, call me and I'll walk him through it on the phone.
-Lord Valve
Dr. Nuketopia said:
I recommend cutting the old leads off at the cap body,
then pigtailing the cap onto the old leads and soldering. I don't do this, but
I have special soldering equipment to vacuum out the old solder in the eyelets
and I lift the boards to prevent building up solder warts under the board. MANY
amps I've worked on have had poor tech work done on them over the years and
have had huge balls of solder run under the eyelets.
Snipping and pigtailing onto the old the leads isn't as "pretty" but rest assured, the electrons don't mind a bit.
Dave Curtis added:
Agreed. There are a few ways to do it:
Loopy:
Cut the lead with enough length to make a loop. Loop the new component lead through the loop and solder. Some would advocate making the joint mechanically solid before soldering, but how solid are all those eyelet connections?
Spiral SpliceŽ:
Another way is to take a piece of snipped component lead (or 20-22 ga tinned wire) and wrap it in a tight spiral around a jeweler's screwdriver sized so that all leads to be joined will fit into the splice. Make it 1/8" to 3/16" long or so. Clip the old component lead about 3/8" from the board. put the little spiral on, slip the new component lead into the spiral and solder. If you clip the old component lead a bit longer and the bend it straight up (but not too close to the original solder connection), this method allows you to make your own hollow "turret", which makes for easier swapping later on (tone caps, etc.).
I don't usually splice, but when I do, it's on one of my (non-collectable) amps using the spiral and only if I think it will take less time than doing it "right" (ie, desolering a multi-lead connection). It takes a bit longer than looping, but is easier to rework and looks better (than looping, that is).
WakyAmps then noted:
FWIW, the snip n' solder to leads method is also an excellent way to
alter/repair PCB amps while keeping the labor costs down (cuz you don't
have to pull the board). Easiest way I know to change the bias resistor in
JMP/JCM heads when converting between 6550 and EL34.
Consider replacing the caps as in the question above. Then...
Don't neglect the weatherstripping under the doghouse. New caps are smaller, and if they're not properly secured, you might get fooled into thinking all your preamp tubes are microphonic.
-Dave Curtis
Yup, that's an important thing to check, that the caps are securely mounted. Recapping under the doghouse means making sure the weather stripping is thick enough to hold the usually smaller new caps in place.
-Dr. Nuketopia