DC on Pot - From Relay?

[jhaas]

In the attached schematic, the DPDT switch shown is actually a 5V Omron Relay.

I noticed scratchyness on the Gain pot. I checked and sure enough, I measured -0.5V on the pot when in "Crunch" mode. In the "Clean" setting, it goes to 0, so it's not leaking thru the .022uF coupling cap. To me, this means it must be coming from the relay.

Is this normal, and if so, what is the best way to prevent the DC from getting from the relay to the pot?

-John

[ChrisM]

I seriously doubt the DC on the pot is from the relay.
I'd put a nice big coupling cap before that pot and be happy, but that's me.

[mbeldyga]

Have you got coupling cap after last gain stage in your crunch circuit?

[Merlinb]

I noticed scratchyness on the Gain pot. I checked and sure enough, I measured -0.5V on the pot when in "Crunch" mode. In the "Clean" setting, it goes to 0, so it's not leaking thru the .022uF coupling cap.

In Clean mode the pot is grounded, so it probably is the cap that is leaking.

[Firestorm]

Merlinb is most likely right. There's also an outside chance it could be the tube. There is always a bit of current flowing in the grid circuit and it can vary widely from tube to tube (0.5V is an awful lot to develop across a 1M pot, but maybe a function of your DMM settings?). Perhaps there's enough gain downstream to amplify the normally small DC "scratchiness" to the point of audibility.

[jhaas]

Have you got coupling cap after last gain stage in your crunch circuit?

Yes, off the page to the right, the signal enters a Fender style TMB tone stack.

In Clean mode the pot is grounded, so it probably is the cap that is leaking.

Good point. The first thing I did was replace the Orange Drop that was in there with a M150, and the problem persisted. Not that that rules out a leaky coupling cap, but I would think the likelihood of them both being bad would be slim.

I'll try swapping tubes, and see if that makes a difference.

Thanks for the suggestions,
-John

[jelle]

My guess is that when you remove the tube, the voltage on the pot will disappear as well.