Peavey 6505/5150 clamp circuit - whatzit do?

[ggjgjhkihkjhiuyiy8788]

Hi everyone. I have a 6505 combo that needs repairs and mods and had a simple question - what is the purpose of the 'clamp' circuit (Q1-J174 and associated bits of the switching circuit)? I take it CR1 & CR2 are doing the actual voltage clamping, keeping the signal between -23 and +23V to prevent overloading, so what is J174 doing? Is it just one of those systems that prevents pops when you switch channels or is there more to it? And why have it at the input to the reverb circuit as well (Q4-J174)?

Peavey 5150 EVH Combo.pdf

Q1 was killing the audio signal to V3b and the fx loop, etc. so I clipped it for the time being and then got thinking about why it's there to begin with. Channel switching seems fine without it, as do all other functions (except the footswitch, which I don't have on-hand to test) so why bother putting a replacement back in? I'm sure Peavey has good reason - I just don't see what it is.

Unfortunately, it's not the only issue I'm facing here - the Lead-Pre causes squealing/oscillating when set >5 with no guitar connected. Yes, that's more gain than anyone would use, but it still shouldn't happen. It's not a noisy/bad preamp tube, it happens with the Lead-Post, tone controls and Presence on zero and a 10pF cap across R9 (V2a) and R12 (V5b) made no difference at all. Swapping V1 or V2 for a 5751, 12AT7 or 12AV7 did nothing and a 12AU7 just makes it happen at Lead-Pre >7 instead of >5. Tried a bunch of other troubleshooting that I won't get into, but I'm suspecting parasitic coupling within/among the first four triode stages.

Any advice or insights would be most appreciated!

[Enzo]

Your guess was off the mark. The two diodes just prevent the signal path voltage from exceeding 24v, they are not the clamp though. This is a high gain amp, and when channel switching, relay contacts transfer and there is potential for loud noises when it switches. Look at the Q7 circuit. The trigger signal comes from the channel switching section - the relay controls. That pulse triggers CR23 triac, which grounds Q7 gate, allowing it to turn on. Q7 is right across the signal path, so when it turns on the signal is shunted. Timing cap C55 controls the length of this shunt - it is just a brief moment.

If you remove the Q7, you have POTENTIAL for loud noises at channel switching. If your amp gets away with it missing, so be it. But that is why it is there.

[ggjgjhkihkjhiuyiy8788]

Thanks Enzo. I think I get it but one other question - keeping in mind I'm talking about the combo version, not the head - what's the purpose of Q12? It doesn't exist in the head-version of the 5150, just the combo as far as I can tell.

[Enzo]

Ah, you did say combo, but not in the title.

OK, the mute is Q1, but the circuit is the same. Q4 mutes the reverb at the same time, Q1,4 have gates tied together, so if one of them fails it affects both.

I suspect Q12 is a power up mute.