Not sure about the hum with a butt-set, but across the 70 volt line, there may just be a lot of residual noise. I would suggest hooking a speaker up instead of the buttinsky. Use the Com and 8, now thinking that might not have an 8 ohm out, but the 25v out should be wanting to see about a 16 ohm load at 35W. Those are really reliable amps. Still being made but now a 35B. Or if you have a speaker with a 70v xformer, hook that up instead. If you still have the hum, I would suspect that one of the power supply filters has crapped out, although not a big deal to repair.

RUSS K.