I'm trying out a Vellman (Dynaco) Stereo 70 tube amp, with integrated volume control from a local shop and I have a hum from my speakers which does attinuate when you raise the volume control with ot without a source connected.
I have the (I believe Integrated Amp since I have to use my pre-amps tapeout loop for connection to the amp) powered through a power line conditioner (tried without too) and sitting in my rack under the other equipment (3rd shelf down).
I have tried to connect a seperate ground from chassis to ground and no change.
3 of the EL34 tubes look the same and 1 is silghtly taller so I'm guessing its different.
I tried to connect the amp using my pre-amp out but this leads to the hum being heard in the next room and the volume control can barely be used.
If I connect with the tape out loop I can use the volume on the amp and it sounds wonderful and 1/4 volume is rockin' the room, but I know that hum is there very low in the background.