Are you sure its not the components upstream from the amp? You can check this effect by simply removing the input connections from the amp, and unplugging all of the gear that is upstream.
I have seen ground loops / ground problems cause this...
If it is the amp -- it seems like a power supply issue -- this may happen due to a bad diode, regulator tube, or capacitor. There are certainly other things it could be - these are the most common ones I have run across...
Good luck -- and follow Sean's advice if you can't quickly figure it out...
I have seen ground loops / ground problems cause this...
If it is the amp -- it seems like a power supply issue -- this may happen due to a bad diode, regulator tube, or capacitor. There are certainly other things it could be - these are the most common ones I have run across...
Good luck -- and follow Sean's advice if you can't quickly figure it out...