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I've been tinkering with the amp tonight and I still don't have a clue how to fix it. Looks like there's some bad interaction between V1 and V2 that starts above a certain preamp volume level. I'm pretty sure that V1B is misbehaving as well as the sine wave shape changes a bit, but it's not clipping. I guess the driver tube is the culprit. Disconnecting the reverb tank input immediately cleans up all signals. With no load on the reverb transformer secondary I can get a clean sine wave at the V2 plate. I even tried unscrewing one side of the reverb transformer and moving it around a bit, but it doesn't seem to have any effect on the reverb driver distortion.
The amplitude at the driver input is now actually much smaller - I previously operated the amp with the EQ lifted and I forgot to tie it back to ground.
I guess I'll start over and try to re-bias the V2 again, increase the value of the grid stopper and see if adding a small capacitor between the plate and the cathode improves the tube stability.
The amplitude at the driver input is now actually much smaller - I previously operated the amp with the EQ lifted and I forgot to tie it back to ground.
I guess I'll start over and try to re-bias the V2 again, increase the value of the grid stopper and see if adding a small capacitor between the plate and the cathode improves the tube stability.
