I have an ST-70 purchased on E-Bay about a year ago.
Recently one of the power tubes started to glow red and output a hum in left channel that would get louder.
So I concluded the old tubes are going bad and ordered replacements.
Replaced with 4 matched Mullard re-issue EL34 tubes and now the amp has an intermittent 120Hz hum in both channels.
To describe: Turning on power, it will first be very quiet as it warms up for a minute or two, then the hum turns on, not loud but noticeable.
Then the hum turns off again about 30 seconds later (sometimes longer sometimes less).
Minute or two later hum turns on again. Like something is turning on/off.
No connection to pre-amp, pre-amp connected, music signal on or off makes no difference.
I can not figure this out, especially why it would come on and go off again.
I've tried relocating the amp to other areas and makes no difference, so do not think it electrical interference.
Checked the bias setting, it is correct at 1.56v. Tried swapping tube locations, no difference.
I can't find any loose connections.
If I put the old tubes back in the hum does not happen.
They are 6CA7 tubes which may be the difference but I was expecting the EL34s to work as well or better.
Any insight or suggestions would be much appreciated.