Almarg and Ghosthouse,
I may be hunting a similar issue, at least the descriptions sound similar to me.
I have an amp that has developed a hum in one channel only. It is this channel because I have isolated it by disconnecting all outputs and inputs. The hum is there as soon as the amp finishes the tube warm up phase and the outputs come on line. I am now running biamped and this hum is greatly amplified when I turn on the other amp. Sounds like 60Hz is bleeding back somehow.
I added a ground lift plug to the pc on the humming amp and that pretty much eliminates the hum with both amps on. The pre ground is already lifted to play nice with my phono stage. So good, so far.
I also run a pair of plate amps to power my bass horns. When I add the plate amp to mix on the same channel as the amp hum, I get audible 60Hz hum again. Lifting the ground on the plate amp does not solve the issue.
I also measured the open circuit hum at the speaker terminals, and with open circuit at the amp inputs.
Hummer: 1.2 volts left channel (grid voltage is about 10mV higher on this channel also)
.9 volts right channel
good amp: .6 volts right and left
Any ideas on how to further track this down?
I may be hunting a similar issue, at least the descriptions sound similar to me.
I have an amp that has developed a hum in one channel only. It is this channel because I have isolated it by disconnecting all outputs and inputs. The hum is there as soon as the amp finishes the tube warm up phase and the outputs come on line. I am now running biamped and this hum is greatly amplified when I turn on the other amp. Sounds like 60Hz is bleeding back somehow.
I added a ground lift plug to the pc on the humming amp and that pretty much eliminates the hum with both amps on. The pre ground is already lifted to play nice with my phono stage. So good, so far.
I also run a pair of plate amps to power my bass horns. When I add the plate amp to mix on the same channel as the amp hum, I get audible 60Hz hum again. Lifting the ground on the plate amp does not solve the issue.
I also measured the open circuit hum at the speaker terminals, and with open circuit at the amp inputs.
Hummer: 1.2 volts left channel (grid voltage is about 10mV higher on this channel also)
.9 volts right channel
good amp: .6 volts right and left
Any ideas on how to further track this down?