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I'm in the process of getting parts together to build an integrated Mark III and PAS 3X. I have a schematic for a modded Mark III and a PAS 3X. I've mostly repaired antique radios, and now want to try making an audio amp. I tried designing an OTL with 12-6AS7s, but never could get it to work.

I plan on using an Antek 4TK400 power transformer and a set of EDCOR's CXPP60-MS-4.2K output transformers. For tubes, I was looking at JJ KT88s, 5U4s for rectifiers (I have about 20), but I'm lacking 12AX7s. I've read alot about the 12AX7A, but not about the 6AV6 being used in audio, I have plenty of 6AV6s and 6AT6s.

I talked to our local metal fabricator and got a price to make the chassis. It will cost alittle more to use 6AV6s (two more holes), but not as much as 2 new 12AX7s.

What should I use for a chassis, stainless?
Should I get the 12AX7s or just use the 6AV/AT6?

Rex, I look forward to your project as I was thinking along the same lines. Here is the chassis I was considering...a friend has a forklift I can borrow.

http://www.diytube.com/phpBB2/viewtopic ... be+chassis

First of all you cannot build a PAS 3X. The 3X used special tone pots that are made of unobtainium. So unless you have an old PAS 3X carcass with those tone pots you are sunk.
First ask yourself if 1) do you need RIAA EQ and 2) if you want or need tone controls. If you eliminate those two requirements your integrated amp becomes a lot easier to build.
If you are going to build a PAS 3 are you going to use the new build PCBs?
If you are then be warned that most do not have the parts placement on the board. In the original kits the two boards were prebuilt so the manual has no parts list or placement diagram.
NOS 5751 tubes will make an acceptable susbstitute for NOS 12ax7s and I actually use them in the line amp stage of my two PAS 3Xs. Sovtek 12AX7WAs go in the RIAA board.

If you are not going to use any low level sources then I suggest just building your amp with a rotary switch, extra inputs and a volume pot.

mesherm wrote:First ask yourself if 1) do you need RIAA EQ and 2) if you want or need tone controls. If you eliminate those two requirements your integrated amp becomes a lot easier to build.

I don't have a phono, so don't need the RIAA.
This one would replace a Kenwood 1100U with 7591s driving ESS AMT-1As. I usually leave the tone controls in the middle. So tone controls aren't needed, but I'd like to have them. The speakers still have the original tone controls on them.

mesherm wrote:If you are going to build a PAS 3 are you going to use the new build PCBs?

I'm going to etch my own boards.

mesherm wrote:If you are not going to use any low level sources then I suggest just building your amp with a rotary switch, extra inputs and a volume pot.

I like the rotary switches at triode electronics, 2 pole, 4 position. But not their volume pots. Where could I get one?

http://www.partsconnexion.com/product7387.html

This is the one I usually use. They come in 10K and 100K also.

regarding stainless, i dunno what tools the average DIYer has, but i had a bunch of stainless sheet, maybe 1.5 mm, that i got for free thinking it would be useful, and i could barely dent it with a dremel or angle grinder cutoff wheel. no way. maybe it was just too thick or some oddball alloy, but i can't imagine trying to make ANY holes in it or bending it let alone making a presentable chassis from it.
it went to the scrap yard this summer

Stainless steel is VERY hard to drill for DIYers. You need carbide or at least cobalt drills. Slow and lots of coolant. If the SS gets hot or work hardens you are doomed.
I am having a custom chassis made for me by a guy in Texas out of 0.093 inch thick yellow brass for a "steampunk" amp I have designed. If you want I could give you his email addy. He normally does guitar amp chassis out of aluminum.

I've read a little about ALPS, I'll have to get me one.

I have a couple of books on making tube amps, and they all say to run the filament wires near the chassis to keep hum low. Would brass have the same shielding effect as steel?

Old boy also has galvanized at 16 and 18 guage. He got kinda scared when I asked about 16 guage. He has some beautiful sheet copper too.

Brass is non-magnetic so it doesn't shield the same as steel but aluminum is also non-magnetic. I've made chassis out of lexan before without a problem with hum. As long as one twists the filament wires and keeps them away from low level wiring there should be no major hum issues.
I am using brass for the look. I also need to be able to solder various things on both sides.

Here's a chassis that will work: http://www.yaegeraudio.com/store/default.tpl

I think Tom McNally has used one of these, very nice!

Re the small signal tubes:

6AV6 is equivalent to half of a 12AX7, and 6AT6 is equivalent to half of a 5751. The two diode plates (pins 5 and 6) can be ignored or just connected straight to the cathode.

I like these rather under-appreciated tubes, and the best part is that they are generally dirt cheap.

There are also 12AT6 and 12AV6, though these are in more demand for use in the old style AC/DC 5 tube "AA5" AM radios. The 6AV6 also has 3V and 4V filament versions.

I'll second (or third) the motion that stainless steel is a BEAST to drill and punch. You'll need machine shop quality tools to work that stuff.

HTH

/ed B

I'm going to get the local metal fabricating shop to make a chassis, just basically a top and back. The sides will be blue acrylic. I have to make sure to keep the heat away from the sides. When I finish, I'll get a bottom made. He said about $45 to make the top. Bottom probably wouldn't be more than $10.

I'm really liking copper. Galvanized is $2.76 a square foot, stainless is $15, and 20oz copper is $12. I'd like to get copper clad steel, but they don't have any. Have to get some bracing for under the transformers though.

Decided to ditch the PAS and do a Mark III + Aikido. Simpler, better, and I have lots of 6CG7s and 6SN7s to play with.

Stainless steel is very difficult to work unless you have the right tools. In thinner gauges drilling small holes is ok, use plenty of cutting fluid and the right drill speed, while socket holes will need to be punched. Forget the thick stuff.

Aluminum is absolutely the easiest to work in any thickness, its light weight, you can polish it, paint it, anodize it, powder coat it. Brass is OK also, and polishes real nice.

I don't know why you would consider galvanized, it is difficult to finish nicely, it doesn't take paint well unless you prep it correctly. Galvanized might be a candidate for those stipled glaze or faux patinas.

The stipled glaze or faux patinas will work on anything (prep is key). I picked up a Valspar brochure from Lowe's a few weeks ago thinking it might look nice on a headphone amp chassis I have in mind. You can combine a base color with some metalic overtones or stipling.

Regardless, good luck with your amp.

Thanks Mike, I forgot to ask about Aluminum. Need to call them up.

I have several sets of knobs from older amps. All of them have knurled insets. All the rotary switches I looked at have 1/4 flattened or round shafts. Is there a way to marry the two?