AA964 Princeton Squeal/Chirp on Power Off

[p_wats]

Hey guys,

First post! It was suggested I sign up here over at the BYOC forum, as I've been getting into amp building more and more lately. Case in point, I picked up an old beat up tube amp (looks like a homebrew from a while back) and decided to rebuild it into an AA964 Princeton.

Here's the before (was dual channel at some point, but the extra inputs were gone and the second volume/tone knobs disconnected, also potentially dangerous masonite board and dead electrolytic capacitors)



The rebuild (still a rat's nest, but sounds great!)



Everything works and sounds great with minimal hiss/hum except when I power off. Then there's this quick chirp/squeal/pop that's not too loud, but really annoying. I'm pretty sure I know where it's coming from, but can't figure out how to kill it.

You can probably see the digital reverb in the left corner in the photo above. There's a separate little power supply section (small transformer, rectifier diodes, regulator) giving 9v for that circuit (as well as the relays for the footswitch). The squeal didn't appear until I wired that all together (if I run the reverb off a battery it's fine).

Currently, the power supply for that section is as follows:

Mains -> Small PT with 9VAC secondary -> rectifier diodes (x4) -> 1000uf cap to ground ->9v regulator (78L09) -> 220uf cap to ground

Things I've tried:

- beefing up the filter caps (added another 1000uf before the regulator to no effect)
- moving wires (just in case there's some cross talk or oscillation)
- moving small PSU ground wire to another location

Anything else you guys would recommend? It's not a deal breaker, as the amp sounds great as is, but that shut down noise always irks me a bit.

Thanks!

[Chuck H]

WAYYY too much trouble to trace the circuit from the photo. A schematic would be very helpful. We need to know what's happening to what circuit when the amp is shut off.

[p_wats]

WAYYY too much trouble to trace the circuit from the photo. A schematic would be very helpful. We need to know what's happening to what circuit when the amp is shut off.

Thanks. This is the AA964 schematic:



but like I said, it only started doing the squeal/chirp when I added the extra power supply for the reverb, which switches off and on with the main power switch. The reverb is inserted between the volume pot wiper and the first 12ax7, when it's lifted out of the circuit completely there's still a pop on shut down (instead of a squeal).

[Chuck H]

I'm sure it has to do with the relatively large cap for the new power supply loading and unloading. But without a schematic It's impossible to be sure. Of course the schem I asked for would have been specific to the unknown circuit. I can't look up what you did on line.

One thing you could try would be to parallel the 1000uf cap with a resistor (maybe 220k). Then it has a reference to 0V at both ends even when the amp is on standby or shut down. If that doesn't do it, repeat with 220uf cap.

[g1]

When you say "lifted from the circuit" do you mean physically or electronically? I had the same symptom with a non-reverb fender amp that had been fitted with a solid-state reverb circuit. The cure was shielding the add-on board with cardboard and shielding tape. May not be the same problem as yours is supply related, but it's worth a shot to try relocating or shielding the add-on board.

[p_wats]

Thanks guys! Sorry for the lack of reply, but I got sidetracked by other projects this week.

I'm about to crack it open and give your suggestions a shot though and I'll let you know how it goes!