Ac30 is borken, any help much appreciated!

Cirrus

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!

Heartfeltdawn

What era of AC30 is it? Korg-era Marshall built, CC, older? 

Cirrus

It's a recent HW head, I've swapped some components out to tune it's voicing so I concede I might have broken it through stupidity, but it's been in regular use for months since I last did anything.

ICBM

Hard to say without it in front of me, sorry... could be several things, including a worst case you probably don't want to know about but which can give exactly those symptoms.

Cirrus

I can cope with worst case. @-)

ICBM

Cirrus said:

I can cope with worst case. @-)

Output transformer… maybe.

A partial short will do that - amp cranked, goes to low volume, sometimes either intermittent at first or goes quieter in stages, no fuses blown and everything else appears to work, just not much power comes out. Not easy to diagnose by metering without calculating power-in and power-out to the transformer.

Could be something else though - hopefully.

martinw

ICBM said: 

Output transformer… maybe.

My first thought.....  

Could be something else though - hopefully.

Yep, fingers crossed.

Cirrus

Cool, thanks for the input. I actually replaced the OT with a "classic tone" transformer about a year ago, so I've still got the original. Since it was probably the least amount of return for the greatest investment, It wouldn't be the end of the world to put the old one back in!

octatonic

Cirrus said:

Cool, thanks for the input. I actually replaced the OT with a "classic tone" transformer about a year ago, so I've still got the original.

I would probably have mentioned that in the original post.

Cirrus

octatonic said:

I would probably have mentioned that in the original post.

Possibly. I wasn't sure if it was relevant, but since I replaced it out of curiosity, the old one worked fine, and this one has also worked fine for a full year, I admit I assumed that particular operation probably wasn't the cause. I've also changed out all the filter, bypass and coupling caps and several resistors, but they were all simple substitutions that I've tested for the usual stuff - the solder joints all seem good, the voltages in the circuit make sense with reference to the AC30 handbook I have, so that's why I've taken to the board to get some more input. If you've got any thoughts they're very welcome!

ICBM

Since you have it to hand, at this point the quickest way to confirm or eliminate the transformer is to temporarily disconnect the new one and hook up the old one - you don't need to mount it in the chassis as long as you support it somewhere it can't fall into the circuitry. You'll know straight away if that's it.

Phil_aka_Pip

@ICBM suppose one of the mods he did caused the OT to fail, would that endanger the one he's about to substitute?

57Deluxe

Brian May didn't really play through all those AC30s on stage... they were empty cabs just for show... #justsaying

ICBM

Phil_aka_Pip said:

@ICBM suppose one of the mods he did caused the OT to fail, would that endanger the one he's about to substitute?

Unlikely unless the voltages have been increased drastically.

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).

Cirrus

ICBM said:

Phil_aka_Pip said:

@ICBM suppose one of the mods he did caused the OT to fail, would that endanger the one he's about to substitute?

Unlikely unless the voltages have been increased drastically.

That was my hope  )

I'll give the transformer swap a go. Hopefully that fixes it, if not Sunday's gig will be through a borrowed Marshall Mk2 Lead combo and 4x12 cab which will be a bit of a departure but probably still fun.

guitarfishbay

Cirrus said:

I really am stumped so if anyone has any ideas or suggestions I'd really appreciate hearing them!

Have you tried using science yet?  Might work if the room dimensions are right.

Cirrus

I don't know whether to give you a lol or start a new thread petitioning for the return of facepalms.  Thanks to ICBM and MartinW for offering help.

Cirrus

I didn't have time to do anything tonight other than a brief poke, and I noticed in the p.i. section, the dropping resistor between b+ and the filter cap doesnt have any voltage at all across it. That doesn't seem right to me.

ICBM

Which resistor? What's its value?

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.

Cirrus

Hopefully this makes sense, it's the only model specific schematic I could find. With the power off, the 22K resistor circled reads 23k or so in-circuit. With power on, the voltage reads 340v in the places I've drawn lines to. I've also shown the filter and coupling cap value substitutions I've made in that area.

martinw

Have you tried a new PI valve?

Measure the PI valve cathode voltages.