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The first thing to do is check that both power valves and both halves of the phase inverter are glowing normally - in the power valves there should be an orange glow near the base and at the top, and usually a slight blue-ish glow on the inside of the glass, most easily visible when you switch from standby to on, and possibly in a darkened room. In the phase inverter there should be *two* spots of orange glow at both the top and bottom of the valve. You may need to take the back panel off to see them clearly, in which case obviously be careful not to put your fingers (or anything else) inside the chassis while the amp is running - take out the six upper screws in the panel, but not the lower ones which hold the valve guard. You may have to slightly loosen the screws on the outside ends of the cabinet to get it to come out easily. (Don't forget to tighten them back up after you've finished.)
"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
https://i.imgur.com/41QkFJG.jpg
The next thing to do is swap the phase inverter (nearest the power valves) with the middle preamp valve, and try the power amp input again. If that fixes it, it’s the preamp valve. If it doesn’t, chances are it’s the PI plate load resistor fault which used to be common on them, but I thought they’d fixed now…
"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
So I just need to order a 12AX7 for now. Is there a preferred supplier or version of that valve?
PM me, to let me know your consultancy fee (seriously). I really appreciate your help.
Basically any supplier of modern 12AX7s will do. I would use a JJ (the most reliable) or any Chinese-made one - even if you can find Russian valves now, I don't find them either as reliable or as good-sounding.
"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
I’m not the master amp tech either, I just have a knack for remembering all this .
"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
"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
Top marks to @ICBM.
Out of curiosity, while I'd got the back off the amp, I checked the bias voltage, which was ~75mV. Does that sound about right? There's conflicting info, when I Google it.
I also should have said earlier - if the amp is still factory-set for 230V, it's best to re-set it for 240V which is what we still actually have in the UK - to do that, swap the white/black-stripe wire that's on P4 with the black wire that's on P10, on the left-hand end of the PCB. You may need to undo the cable tie on the wiring bundle, but you can often pull the wires through as it's usually not done up very tightly.
As always, make sure you know what you're doing and that the amp is safely powered off and preferably disconnected from the wall before poking around inside.
"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