Need help with Peavey mute/clamp circuit

[Gaz]

I've been playing around with the channel-switching 'clamp' circuit found in many Peavey amps, but can't get it to work properly.

For those not familiar, my understanding is that the circuit uses the channel/feature switching relay's pulse to trigger a triac and jfet in the signal path to shunt the signal to ground late in the signal path after the switching elements. The jfet is timed for just long enough to mute any pop/click/etc from the relays in the preceding circuitry from getting through, but not long enough to really be noticeable.

Here is an example from the 5150 (2nd page, middle-right):

https://el34world.com/charts/Schemat...y/5150_evh.pdf

I've gotten it to work, however only when the relays are being switched on. It was my understanding that the triac would be triggered from both an on and off pulse, but I can't figure out if I'm missing something critical in the circuit. Should this circuit mute the signal turning relays on and off? Hopefully that's not too confusing - it's hard to formulate this into a question clearly!

[Gaz]

OK, turns out this circuit is really as simple as it seems, and I just had a faulty triac (no doubt from my own overheating on the breadboard!)

For those interested in this topic:

I have to say this is probably the most clever circuit I have come across in a long time (even though it's been around for 20+ years). It is seemingly a cure-all for any noise coming from a switching system. In my case I had a annoying "clink" noise from relays early in a high gain amp's signal path. It was not from DC offset, and I could not get rid of it with layout or using transistors to remotely trigger the relays (which was suggested by others). I also tried a cap across the coil in addition to the mandatory diode, but to no avail. optos fixed it with their soft-switching, but they are expensive and harder to find (plus have poorer isolation when off). The Peavey circuit (also use by may others), only cost a buck or so in parts, and is easy to adjust the timing of. To me the mute time needed to silence the pop is imperceivable. I've heard of this being bad in some amps, but my guess is the popping is really bad to begin with or it's used as band-aid for poor layout or being careless about DC offset. JM2C.

[Enzo]

That one part, the JFET, at the end of the preamp mutes the signal path for a moment. The channel switching though is done at multiple relay contacts and other places. So the one part covers all of them, rather than coming up with multiple solutions for all the sources.

And I do stock the little triacs, but in my experience it is almost always the JFET that fails.

[Gaz]

Yes, I don't know if I mentioned but this was for a homebrew amp, which is why I was having trouble getting it going. The bad triac was either overheated by me.

When the jfet fails, do you replace with the same part (J174/75), or have you found a more robust replacement? Maybe something with a high breakdown voltage

[Enzo]

More robust? They made tens of thousands of these amps, and a FEW of them have a failed JFET. And that even usually after years of service. I see no need to reengineer the one part. I replace them with similar parts.

[Gaz]

Understood! That’s good advice. Thanks

[Gaz]

Enzo, I did have a question about this Peavey circuit - What is the 47R resistor on the TRIAC gate for? I'm trying to figure how to determine the value.

[Enzo]

I never thought about it. I might guess pulse stretcher?

[Gaz]

I never thought about it. I might guess pulse stretcher?

Thanks, from a little more research it looks like it to limit current, but I'm not sure how to calculate it in this application. It appears in all the Peavey schematics, but not in some others that (mostly) copied the circuit. I don't know if it's necessary or it's just something they copied from the datasheet. The MAC97 datasheet shows a 51R resistor on the gate.

[Enzo]

47, 51, what's the difference? Some smallish resistance.