Hummm...

[Ziradog]

OK, that makes sense. Somehow from the previous post I thought you meant to ground the input jacks to the chassis in one place & the buss to the chassis in a second place.

I am about 85% sure that the jacks do not touch the chassis but without having the amp in front of me I can't be 100% positive. I have grounded the rail/buss to the chassis with a jumper without any noticeable improvement.

 

[gadget73]

Yeah I'm making guesses at this as well. Doesn't help a bit when you've never been inside of the amp in question

 

[Ziradog]

Well, we made some progress today. I took three more 6AN8's over, and we found 2 (out of 7!) that gave reasonable voltages. We found no other issues in that stage. We played around with the 2 6AU6's and somehow added some nasty 120 Hz buzz to both channels for a while. Finally fixed that by cleaning the sockets & pins (again) on those 2 tubes. We added the extra filtering stage (47 mfd cap & 100 ohm resistor off pin 8 of the rectifier) that Dave Gillespie suggested. There is STILL a very low-level 60 Hz hum, even with the rectifier tube out. I am debating trying to bypass the front power switch & pilot light, and run the AC line in straight to the power transformer just to see if it helps. For now, I am going to try to find 1 (or 2) capacitors that re physical smaller in diameter than the 47 mfd I used for the extra filter stage, and then give it a spin.

 

[gadget73]

Some of those pentode/triodes had pretty variable characteristics. I think they were mostly intended for TV applications where it probably doesn't matter so much. Frankly it probably doesn't matter all that much in the audio circuit either. I only have one amp that runs that sort of tube, and I got lucky in that the voltages come out pretty much dead-on the schematic and quiet using the first pair of tubes I bought for the thing.