Hi all, I know this is a common question, and I've read through the archives, so here goes: I have a vibro champ clone that has a constant 60 cycle hum. It's a "soft" sine wave type hum, not buzzy. The amp works otherwise. The volume knob makes no difference. Nor does jumpering the control grid to ground, so I don't believe the hum is coming from the preamp stage. It is quiet until the tubes have warmed up, so I don't think it's magnetically induced. I've replaced all three filter caps. I've replaced all preamp tubes, the power tube (a 6V6) and the rectifier tube. The filament leads from the PT are not center tapped, but it came with a positive reference voltage. I tried moving that reference to ground but it didn't make a difference. Leaving the filament circuit ungrounded makes a hiss in the preamp stage, so I don't think that's the cause. Does anyone know of anything else I should try?