Help me
@ICBM @MartinW and all, you're my only hope!
I was hot swapping preamp tubes, then rocked out for a few minutes with the volume right up. The Amp went a bit quiet, then a few mins later whisper quiet. I've swapped out all the valves to no effect, and checked the fuses which all show continuity.
All the control knobs seem to work as they should, the output is just really low.
The voltages on the plates and screens all seem ok, and when I test voltages with the amp on I'm getting a few loud pops through the speakers when I probe some component legs which makes me think the power section's working ok. And both channels are quiet, so I think it's a PI problem.
Sending a signal into the input (all I have at home is a lead so I'm pressing my foot on it to get hum), if I flick the half power switch which increases the power section's cathode bias resistor the hum gets louder then fades a bit. If I flick the switch back to full power, the amp goes silent for perhaps five seconds then the hum suddenly reappears. It's the same if I probe the high voltage sides of the coupling caps on the phase inverter - the hum totally disappears then comes back a few seconds later.
With the multimeter set to read dc voltage, if I hold one probe to ground and touch either side of the eq stack's 47uf cap with the other probe I get insane high frequency oscillations, albeit at the amps massively diminished max volume. I don't know if that's normal but I thought worth mentioning.
I really am stumped so if anyone has any ideas or suggestions I'd really appreciate hearing them!
Comments
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"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
Bandcamp
Spotify, Apple et al
"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
Bandcamp
Spotify, Apple et al
Studio: https://www.voltperoctave.com
Music: https://www.euclideancircuits.com
Me: https://www.jamesrichmond.com
Football is rubbish.
Bandcamp
Spotify, Apple et al
"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
Seriously: If you value it, take/fetch it yourself
Brian May *did* play through three of those AC30s too . It's a wet/dry/wet setup, with a row of backups in case one of them goes down, and another row of backup backups... old AC30s are not renowned for their reliability when cranked as hard as he did and fitted with solid-state rectifiers (which does increase the voltages).
"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
Bandcamp
Spotify, Apple et al
Bandcamp
Spotify, Apple et al
Bandcamp
Spotify, Apple et al
Do you mean the one which feeds the filter cap from where the two PI plate resistors are supplied?
Check the voltage at both ends of the two PI plate resistors.
"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
Bandcamp
Spotify, Apple et al
Have you tried a new PI valve?
Measure the PI valve cathode voltages.