I had an Ampeg B15 that kept coming back because the bias would go wonky and the tubes would red plate. I had replaced all the components in the bias circuit and mod’d it to be adjustable. Ran it on the bench for two days at mostly full power and it worked fine but the owner took it home and after a day or so it went south again. Put in on the bench and the bias voltage was way low, so something was pulling it down. As I was inspecting the traces for shorts and contamination I noticed a pad that I had glossed over previously – a wire that went to the hum balance pot.
Turned out the hum pot was bad and was pulling the bias down – replaced the pot and it’s solid now. Should have caught it sooner, if you look at the schematic you can see the ‘x’ that leads to the the hum pot. That said, this is the only amp I know is configured like this.