The Amp Garage

Ok, then the issue is somewhere in the PI circuit (you mentioned that pulling V3 makes the amp quiet). Is the NFB reconnected ? Can you measure the V3 voltages again: pin 1 and 6 pin 3 and 8, junction of the 4 resistors, junction of 24k and 390R/4k7 What happens if you ground the 0,1 PI cap on the 3...

Nope. The other terminal of the cap.

grounding either node kills the hum completely.

you mean at the PI caps ? or the jack tips ?

Confusing...

but not like when I ground out the PI 0.1uf node which kills it stone dead.

Really that one ???
Martin asked you to ground at the PI entrance cap (0.02uF)

Marcus, grounding the PI entrance (connection blue wire to 0.02uF Cap) cancelled the hum, right ? Grounding lug 2 of the MV potentiometer (or just turning it all the way down) doesn´t have an effect on the hum. IMO the source of the hum must be between those parts. ground the tip of the send jack - ...

Wiring of the jacks looks correct. Try with a patch cable to exclude a bad switching contact.

Marcus, do you have a black wire between the grounds of the jacks ? I can´t see this on the pics. If there is one, cut it and see (hear) if that changes something.

Yes, in the layout the V2 cathodes are wired to ground 1 in the schematic (#124 1.7) they are connected to the PI ground, that´s correct.

Ok, then it must be the jacks since turning down the MV doesn´t affect the hum

In the layout there´s an additional wire connecting the sleeves of the jacks (grounds). With non-insulated jacks there is already a connection via the chassis. Or in other words: this is a ground loop.

having a cable in the send return stops this somewhat That´s strange, isn´t it ? Are the jacks insulated to the chassis ? The PI/NFB ground wire goes to the return jack and to the send jack ? Or only to the return jack ? Just to clarify: In your amp the send jack is the one near the speaker output,...

The polarity of the NFB cap is correct. Did you ground the 390R and the "-"-terminal of the NFB cap ?

Yes, some better focussed pictures could help.

The 22M Resistor from treble 3 to the R/J switch is very close to the treble pot and there is no isolation around the leg. If the leg touches the housing of the potentiometer the PAB switch would have no effect anymore.

I have clipping when playing hard that stops when I insert a cable from the send to return Could be a bad solder joint at the send and/or return jack. In the schematic the switched jack is the return jack, not the send jack. That offers a signal access without interrupting the signal path to the po...

A new day with new problems... the hum now is volume dependent you mean the preamp volume potentiometer, right ? (not the master volume). I have to think about it... Another point: The input wiring seems different from the original. You omitted the FET circuit. I can see two grey shielded wires runn...

AFAIR you have another (D-style) amp. If it has an "effect loop" (preamp out / poweramp in) you could cross check the #124 to exclude that at least a part of the hum is derived in the preamp section (preamp #124 patched to poweramp other D-style amp) @lars, the heaters have a center tap so no need t...

I'm wondering if the general loud hum is in fact constant motorboating and I've got my OT primaries the wrong way round? can that be a symptom or is it always the loud howl? Swapping the primaries is worth a shot Maybe. You may just desolder the yellow NFB wire from the speaker output. That is the ...