If your question isn't answered in any of these FAQs, ask it on alt.guitar.amps !
Holesaws are fine, as long as you use them at lower speeds and apply some cutting oil. (Believe it or not, bacon grease is the preferred lubricant for aluminum.) A lot of metal particles will be generated with a hole saw, so you may want to consider taping a piece of paper behind the hole location to catch all the crud. That "sawdust" can wreak havoc with the rest of the amp. Another option is the Klein Unibit, a stepped rotary cutter which can give excellent results. I've used them for years in my shop. This bit really needs to be used with a drill press, and the work needs to be clamped to the table. A drill press with a powerful motor will swing that chassis around and beat you with it if the bit siezes. I use the Unibit for most smaller holes (under 1/2") and I use it when I don't have a punch the right size for a larger hole. Which brings me to the best tool for the job, a chassis punch. Greenlee makes the best ones. A chassis punch of tube-socket size is going to run you $25-$40 for the non-ball-bearing type, depending on where you buy it. It'll produce a great-looking hole with a minimum of fuss. If you anticipate needing to make this size hole in the future, invest your money in the right tool. You won't be sorry.
-Lord Valve
Stepped unibits will work pretty well, or a Greenlee chasis punch will do a great job as well. Sescom (www.sescom.com) offers some english made punches which are about as good as the greenlee for a bit less money.
To drill a chassis for a tube socket, first measure the hole pattern of the socket you have in hand. Then draw a fine, straight line where you want the screw holes to line up. Then mark the center points of each screw hole and right exactly in the middle the center point for the socket hole. Use an automatic center punch to make a dimple in the metal at the dead centers of where the holes are to be. This will be the only way to get an accurate drillout where the hole is supposed to be if you use a hand drill. Clamp the work and drill the screw holes and a pilot hole for the unibit or chassis punch. Use low RPM for aluminum and I find it best to drill the two small screw holes first. After that, use the punch or unibit to make the socket holes. Deburr it all with a dremel tool.
-Nuke
If you don't have the right size punch, a reamer is handy.
-Tony Hwang
Be careful what you get, Greenlee makes Chassis punches and Conduit punches, and there is a difference in size. For example, a 3/4" Conduit punch actually cuts a hole 1 1/16" in diameter. A 3/4" Chassis punch cuts a 3/4" dia hole.
-Casey4s
Lubrication isn't as crucial on aluminum, but always use it when drilling or cutting steel. Your tools will last a lot longer (less heat stress), it's much less likely to bind, and it helps cut down on the flinging of meta slivers. Doesn't hurt on other metals, either.
-Miles
Greenlee makes square (and other shape) punches. I've always drilled an appropriately sized round hole and used files or a nibbler to create small non-round holes.
-Lupi
Be careful what you get, Greenlee makes Chassis punches and Conduit punches, and there is a difference in size. For example, a 3/4" Conduit punch actually cuts a hole 1 1/16" in diameter. A 3/4" Chassis punch cuts a 3/4" dia hole.
-Casey4s
Lubrication isn't as crucial on aluminum, but always use it when drilling or cutting steel. Your tools will last a lot longer (less heat stress), it's much less likely to bind, and it helps cut down on the flinging of meta slivers. Doesn't hurt on other metals, either.
-Miles
Casey4s (David McClain) has a tolex tutorial at http://y42.briefcase.yahoo.com/bc/casey4s/lst?&.dir=/&.src=bc&.begin=9999&.view=l&.order=&.done=http%3a//y42.briefcase.yahoo.com/bc/casey4s/lst%3f%26.dir=/%26.src=bc%26.view=l .
Go here: http://www.web-tronics.com/. Click on the model 9300G handheld DMM. I have one of these (I'm a meter junkie; you'd puke if you knew how many I have) and I've been giving it a workout in my shop for the last month. It's got large digits (nearly an inch) and the display is crisp and easy to read from far away. Amazingly jitter-free, considering the price. I'm flat out tellin' ya this little bugger will pee on any Rat-Shack meter that sells for three times the price. The price? Nineteen bucks! Don't be tempted to order the next one up ($29) because that one ain't so nifty. I have one of those, too, and while it has more ranges and does a whole lot more stuff, it's not as easy to read or as stable as the cheapie. The cheapie has better leads and looks cooler, too. It'll resolve a tenth of a millivolt, so it's ideal for setting bias by the cathode resistor method. I recommend this one to all newbies; if you screw up and fry it, you're only out $19. Damn near disposable. For you guys who want to get something classy and not shell out a whole lot of bread, eyeball the new Fluke model 110, at $99. (No discounts anywhere that I could find, so get it from a killer outfit like: http://www.metersandinstruments.com/. I have two of these, and I'm tickled pink with 'em. Little bitty things, too, so they pack in field kits really well.
-Lord Valve
There is a tool. I looked like that caveman seeing a fire started when I first held one. The brand name is Heyco, not surprisingly they also make the grommets. One hand, squeeze, insert, done.
I did a BUNCH of 'em before I found that thing. I eventually got pretty good at it, that's eventually and only pretty good. Use a small pair of slip-joint pliers, or the biggest needle nose you can find.
If anyone goes to buy a pair, they come in two sizes, large and small. The only difference is how the toothed jaw is set. So only buy one and reset it for the really big ones. Just like tuning a wah pedal.
-Ron Sonic (from three posts)
I've always removed those Heyco strain-reliefs fairly easily using some 'pump' pliars (aka "slip-joint" pliars).....with the jaws adjusted for 1/2" or thereabouts, you can squeeze in just the right place, with parallel-face jaws, and grab (and pull out, or insert easily) those striain-reliefs.
-Greg H.
I've done that, too, but with channel lock pliers I've ended up with "tooth marks" on the strain relief. I'd imagine the nylon-jawed Cannon plug pliers we used on aircraft would do the job nicely.
-Monte Barnett
They also make smooth jaw pliers "no-mar" pliers for plumbers.
-John King
Takes a bit of practice... better to grab the strain relief from both sides with needlenose, and gently work it out the back side while compressing it.
-Steve Cowell
If it's just pooched up or torn here and there, I use a hot glue gun. Otherwise, contact cement works well for larger areas.
-Lord Valve
Zip-Strip will take spray paint off of any Tolex or similar cabinet covering without hurting it. Get the regular-strength formula, not the heavy-duty stuff. You'll need plenty of wet rags to take it off with - make sure you get it all off. A toothbrush will be helpful in removing the paint from down in the grain of the vinyl. (If a toothbrush won't do it, get a fine-bristle brass [NOT steel!] parts cleaning brush from the hardware store.) It may take as many as four (or even more) applications to remove it all. Be patient, give the Zip-Strip time to work. Don't use Goof-Off, it's nasty and it'll eat the Tolex up.
-Lord Valve
You will note that on the new switch, the terminals are numbered 11, 12, 24, and 25. Pins 12 and 25 are connected to the incoming line, and pins 11 and 24 are connected to the load (power tranny). If I'm not mistaken, the old switch uses the same numbers, so you can just note the terminal numbers the wires are connected to on the old switch and connect them to the same terminals on the new switch. Once you've done this, you will be left with an extra wire. You can either tape it off (or insulate it with a small piece of heat-srhink tubing) or track it down, cut it off at the other end, and pull it out of the harness. BTW, if you get the wiring backwards (line wires on load terminals) the light will stay on when the switch is turned off. I don't suppose I need to mention that you should make sure your amp is UNPLUGGED before you go messing around with the AC switch wiring, do I? Good, I didn't think so. ;-)
-Lord Valve
I tried samples of various Minwax products too and I went with orange (ie, amber) shellac followed by a topcoat of nitro lacquer.
In comparing real aged tweed to the "fake" stuff, the problem is the shellac and all are not really red/orange enough. Also the modern tweed seems to be a bit too green, so the tint goes more brown than the nice orangy color that real aged tweed does.
Here's what I'll advise:
Start with fresh, clean new tweed (or clean it darn well first). Tape off everything you don't want paint on and disassemble the cabinet.
Use a clear shellac coat first, about a 1lb cut. (1lb flakes/gallon). Let it dry thoroughly. The surface of the tweed is very porous and will take up color unevenly unless you clear coat it first.
Then use an amber/orange shellac, also in a 1-1.5lb cut with a brush. Build up the desired color, but seriously, don't go too overboard with it.
Once you get to the right color, let the shellac cure for at least 24 hours.
Apply a satin or gloss nitro lacquer top coat with a spray gun or use a nitro in a can spray. (optional).
If you intend to use the nitro, use a premium quality dewaxed shellac flake. If|you don't want to do nitro, then you can just use the plain old Zinsser amber and clear in a can. CHECK THE EXPIRATION DATE ON THE CAN!
This place sells all the stuff you need:
Get the premium de-waxed shellac flake, the anhydrous alcohol and the nitro in a can.
Remember do NOT try to duplicate 40 years of aging, you can't. You can get a nice tint though and it looks a lot cooler than bright-yellow green new tweed.
I haven't yet figured out how the original tweed was coated. The old tweed coating is readily disolved with alcohol, which leads me to think shellac, and not nitro, which is more resistant to alcohol.
I also have no idea what the yellow coating is on the new tweed fabric. It seems resistant to everything, water, solvents, bleach even. I'm not sure whether the original stuff had any coatings on it when it came from the mill.
-Dr. Nuketopia
Rob, you want to be a good boy and switch the B+ on this amp. Reasoning is, this is a fixed bias amp and when you lift the center tap you also kill the bias supply. With fixed bias, we want the bias supply up before the B+ hits the tubes.
You will get some difference of opinion on this. Some have argued that it's not an issue with a tube rectifier. I think you will find the consensus finds it of concern, and most designers will not lift the center tap with fixed bias. You will find even more difference of opinion on pre or post filter switching. We switch the center tap on cathode bias amps. This is an excellent method for cathode bias, switching is silent and a bit easier on the switch. Considering the layout of the amp in question, I would recommend using the ground switch and switching the B+ tweed style, right off the rectifier tube. You do have a power cord with ground installed and ground switch disabled, don't you?
-Rick Sisney (OKRick)
I don't bother with mull-caps anymore unless there's absolutely no room for singles.
-Amp Medic
I agree on this. Back when I used to do esoteric hi-fi, we were constantly tweaking Dynaco products including the Pas-3 preamps. We found that multi-section caps seemed to have some from of coupling between the sections resulting in low level hum and noise. I was able to audibly reduce this by using discrete caps rather than muti-sections. I also found that by placing these caps physically closer to their actual circuits (like right on the individual boards) it reduced noise and seemed to smooth out and improve bass response (especially in the phono preamp stage). The only muti-section caps I regularly use these days are the dual types used in Marshalls. I would never consider using NOS filter cans [in other applications] as I would not consider them to be reliable enough for my customers.
-Matt Seniff
If you just want to maintain vintage appearance, leave the can cap in place, and use individual caps underneath, where nobody but another tech will see them. Just remove the old cap leads.
-Miles
The transformers I worked with had surface rust only. I won't bother with rusty messes. I would not use naval jelly on laminations as there is a risk of it getting into the laminations. Naval jelly is jellied acid and if you don't neutralize it with water and/or baking soda it can contaminate the laminations. Not worth the hassle in my opinion.
The epoxy I used is 3M Scotchcast 265, which is a powder that you mix with acetone. It is very thin and dries quickly. Being so thin, it easily seeps into the gaps in the laminations. It won't take too much vacuum to expel the air and drive in the epoxy.
I happen to have an old vacuum chamber at my work that was designed for outgassing potting materials (you don't want air in potting intended for high voltages > 20 KV). It is about the size of a beer keg and with a wall thickness of about 3/8". Based on the noise the pump makes, I'd estimate the vacuum is less than what a standard refrigeration pump would do, but definitely more than a vacuum cleaner. Actually a good shop vacuum with a 2 or 3-stage blower would likely be ok, but it would take longer.
To conserve the amount of epoxy I used, I built a wooden box that closely contours to the transformer and lined it with a thick plastic bag (making sure it is not attacked by the epoxy/acetone). I dropped in the transformer, filled it with the epoxy mixture, and placed it in the chamber. 5-10 minutes under vacuum and you take it out. Let it drain and dry overnight. I check everything on a hi-potter, but note that the added insulation may actually increase dielectric breakdown strength.
(I asked some of the [folks] on the line what I may have missed-- I forgot to note that best results include a post bake of about an hour at 250-300 degrees F in the oven. If the transformer is vintage with dry wires, I'd use about 225-250 degrees.)
I'm sure this group could dream up better procedures, but this was quick for me. Now that freon is outlawed, I know you can buy refrigerator service pumps at garage sales for a few $$. Maybe someone could get develop a small business refurbishing transformers....
-Matthew Honnert