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Comments
Does it hum with all controls at zero? If so it could be a biasing fault. I would have to dig out a schematic to tell you the sense resistor but it will be a 1W 1R with a diode across it.
If it is biasing and that is not a duff EL34 then it could get deep and you will need an informed tech.
D36, R 217, TPs 10 & 18. Do not measure to chassis, should be 25mV
Dave.
Set the amp up with the cabinet face down and the chassis the right way up on the back of it, and wait for the fault to develop - you should then be able to find it by carefully moving the components. You may need to tilt it to the angle where it comes and goes if it’s very sensitive.
If I had to guess I would put a tiny bet on a filter cap joint.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Good plan. You can also remove all the valves except the output pair, if the fault still occurs you know it is in the OP stage or possibly the MOSFET PI though quite how the latter would cause hum defeats me at the mo'!
Dave.
A gentle thump or probing with an insulated object will help you find the offender.
Power valve?
Dave.
Yeah power valve, lay it down and it worked, stand it back up and it hummed.
I live and learn
I’ve come across quite a few, and even fixed some of them! Mostly JJs.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein