Very strange hum/buzz

[Bob-I]

I'm doing another transplant, Peavey Deuce -> Dumble complete with effects loop. It's working out very well, cleans are sparkly and fat, OD is ok, I'll tweak it for better overall OD but when I connected the loop I've having very strange issues.

The loop connects directly after the OD relay, just before the PI. On the cleans it's fine, nice transparent parallel loop just like I used in my external "Starr-U-Lator" and in my 3 channel amp.

I flipped to OD and same thing, at #1 on the master. As soon as I hit #2 a terrible hum/buzz jumps out. I took out a chopstick and starting moving wires, when my hand comes over the input side (CF side of the 12AT7) the hum goes away. Over the input cap, the output to effects cap, tube socket, even the OD master. Take it away, hum returns.

I shielded all wire going in and out of the OD and to the loop. The loop connects directly to the PI via the coupling cap.

I'm stumped.... but I've found that everytime I get stumped someone here has the answer within 24 hours. Lots of folks far smarter than I am.

Thx in advance

[odourboy]

Where are you getting the B+ for your loop?

[Bob-I]

Where are you getting the B+ for your loop?

I connected a 10K / 22uF node off the screens

[odourboy]

Just thinking out loud here.... Since the CF doesn't provide any gain, I'd be looking for feedback occuring in the return amplifer. Since it's affected by the level of your master, I'd start by turning up the 'Send' level and turning down the 'Return' level an see if you get stable operation at higher volumes.

[Bob-I]

Thinking out loud solves many problems. You got me thinking about the B+. I forgot to measure/calculate it and just put in 10K on my layout. Turns out the B+ was WAYYYYY too high, 425V on a 12AT7. I increased the dropping resistor to a 220k (too much drop now) and the buzz is gone.

I'll have some adjustments to do on the dropping string but this solved my immediate problem.

Thx for taking the time to look.

[heisthl]

It will be quieter if you remove the presence and loop grounds from the front buss bar and ground them to the B+3 and B+2a cap grounding points respectively.