abe:
the yamamoto is a 45 tube amp and it is tube rectified. i have a few 80 tubes that i can roll to see if that reduces the hum. i emailed kevin covi and he seems to believe the hum is caused by the 101D tube as when i switch the amp to mute, the hum disappears.
I see! If that is the case, ask Kevin if the 101D's filament supply is AC or dc. If ac, maybe he can say if its possible to convert it to DC (by means of adding an SS rectifier inside the chassis). If that is the real source of the hum, using dc filament supply will knock down the hum at least 90 to 95%. IME!
regards,
Abe