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Does anyone know of a 6GH8 style of circuit board for the Mark 3? All I seem to find are stock 6AN8 boards. It seems that 6AN8 tubes are drying up and I like the options with the 6GH8 pinout.

I might try making an aluminum plate and P to P wire the circuit for 9AE and 9DC pinout tubes. I've done this to a Heathkit AA-151 and used 6BL8 tubes and worked out really well!

I use cables with 90 degree ends and route it underneath.

I'll throw in a vote for the Panasonic caps, they sound very good! They also have a box version now, looks good on a pc board if you use one.

The 7687 is the same pinout as the 6GH8, as far as I know the 7199 is all by itself. I sold the Corvair a couple years ago, now I have a low mileage original '95 Impala SS.

There are two "premium" pentode - triode tubes I know of is the Sylvania 7687 and the European E80CF. The 7687 was supposed to be a 7199 killer and the E80CF was a uprated 6BL8. Both are plug & pray if you have a 6GH8 pinout. The 6AN8 is a great sounding tube and there were JAN military out...

Those are excellent units, I had one years ago. You will need to run 7189's in it as the B+ runs high, and have a million caps to change.

Don't forget there's the K-502 amp kit from AES that uses the 11BM8 tube. Same tube, different filament voltage. Then there are others too, 8B8, 16A8, and 50BM8.

That bubbly transformer is perfectly normal and should test out fine. Dyna probably switched suppliers and that's what you got.

Look on the bias pots and filter cap can for a seven digit number. the first three are the EIA code and last four are the date code (week/year). That will give you a ballpark date.

Yes, and they sound excellent! Did it on a pair of MK-3's I built around 10yrs ago. Driver circuit is a copy of the Eico HF89.

A few facts to remember is these things are 50 or more years old, they were designed / sold to a price point (inexpensive), and were designed around the technology of the time. Over the years I have restored several of '70s and found that replacing just this and that ain't gonna work anymore...

I've built a couple EL84 based amps with AA-100 iron. What I did was used a R-C input filter to tame the B+, around 50r to 100r, ended up being R-C-L-C. The rest was a typical circuit similar to a Dyna ST-35. They sounded really good too!

If you want to get creative and play with the heater voltages there's the 5V6 and 12V6. These are identical to a 6V6 except for heater ratings and NOS can be had for very cheap! There's also a locktal version of this tube that is cheap too.

1/2 watt is all you need!

That is definitely RCA. FWIW, it's identical to a 12AT7 except the filament arrangement.

Does it have "gold" plated tube sockets? If so, replace with normal sockets. The gold is microscopic thin and delaminates causing all sorts of bugs. This is something I found out the hard way!

One thing that will help the sound and safety line operated of amps like these is always use an isolation transformer!

I used to have the Carver SDA-490t, loved it, lasted a long time until the laser started getting weak. I got curious about that tube circuit so I reverse engineered it. Looked similar to a PAS preamp but with a lot of feedback so probably little or no gain. Only 45v or so on the plates...

Three things you NEED to do NOW, Replace the coupling caps! Replace the selenium bias rectifier with a 1N4007 diode. The 5U4 tube needs to be replaced with a 5AR4/GZ34.

What about the Edcor XSE series? I've used these in several builds and very pleased with the sound and at $20.01 they are a bargain! https://www.edcorusa.com/xse15-8-5k Here's a RH-84 I built into an old Heathkit AA-32 chassis with the transformers mentioned. I use it daily as my desktop...

I've been using Belton sockets, they have thicker metal and grip the pins tight. Almost all ceramic sockets are China made and quality is fair to poor. NEVER EVER USE GOLD PLATED SOCKETS! The gold plating is very thin and delaminates causing all kinds of noises, poor connections etc.

Yep! It keeps the .22 cap from building up a charge. HTH!

One other thing you may consider is add a 1 meg resistor across the output jack to prevent a loud "thump" when you select phono on your preamp.

Yes, this is a very nice phono circuit that sounds excellent. The Heathkit AA-151, SA-2, and the mono versions use this very circuit. It works well with 12AX7's, 7025's and 6EU7 tubes.