So I've got a one channel Hoffman AB763 on the bench. Finished up all the wiring, so I fired it up for the first time last night. Pilot light came on fine, filaments lit up, but no sound whatsoever through the speaker. No hum, no hiss, no nothing. It was late and I was tired, so I didn't check voltages or go through the schematic verifying connections or any of that stuff. But does that fact (the total lack of hum/hiss through the speaker) lead you to think of one thing over others? Grounding issue?