The Amp Garage

Not sure what you mean by blow out the pots. Do use a chemical cleaner. Also, your tube tester will not tell you if a tube is noisy.

Seems you are anxious to replace the filter can, which is needed but, the rumbling/noises are not caused by the filter cap. Sounds like you need to properly clean everything. Pots, jacks, sockets etc. The noises are likely a tube issue.

Also, filter caps will not cause the symptom you describe.

The insert contacts are always suspect. Connect a cable from send to power amp in to check.

Are you making "audio" measurements to validate what you are hearing or just measuring DC voltages? Certainly one can assume no production PP output transformers will be balanced. This is interesting stuff.

Those pots do eventually fail. I have replaced the pot (and cap) with a 1 ohm resistor that gives you a convenient location to measure bias current.

In this schematic??

OK, thanks. Never would have guessed THAT.

What inductor?

Forgive me but what is HRM?

I would assume the messed up board was the result of the hacking to figure out the circuit? Suhr's work is usually impeccable.

Correct, just an example. I did use a stainless steel bolt and never had any issues. The mu shield was grounded with no adverse affects though?

Very happy to meet everyone. My email address is begnater@aol.com if I can ever help anyone.

Any progress yet?

If you determine it is just proximity hum from the transformer, you can effectively shield it with mu-metal. I have done this many times in the past on the TOL100 amps. These used toroid power transformer that were supposed to reduce the radiated field. Ha.... all they did was narrow the field into ...