I recently rebuilt a 1963 Stereo 70 using a DIYTube driver board and have noticed that in the left channel, the bias is very touchy and tends to drift at the slightest provocation. The end result is that either of the two tubes starts to get a little toasty (visibly) and I jump up from my chair with the mini screwdriver and DMM and rebias that channel. The other channel, however, is steady as bedrock. I have not (as of yet) tried swapping the tubes to see if is the tubes or channel because biasing is a PITA, but that's probably the next step.
One additional clue(?) is that the left channel gives a thump on turn-off whereas the right channel is silent upon turn-off. Could one of the coupling caps be passing a small amount of DC and causing the bias to be twitchy (as well as causing the turn-off thump)? Caps are 0.047uF K40Y and 0.1uF Sprague 715P (and are under the board).
Thanks,
-D
p.s. Here's the amp (at an apparently stable moment):