Capacitive coupling?

[GainFreak]

Hi,

I have an amp that has a clean signal leak with all Gain and Volume controls down (2 channels with separate Gain and Volume controls and common TS, PCB less the tubes). It's well audible when it leaks into the drive channel at low gain settings and is very annoying. It looks like the last triode grid (just before a non CF tone stack) is picking up the signal. I know it would be specific for every amp, layout etc but if this is capacitive coupling what general approach and rules would you recommend for handling it ?

[xtian]

Is this a hand built amp with wires you can push around with a chop stick? You should be able to notice a difference in volume as you move wires around, if the issue is capacitive coupling.

[Helmholtz]

General countermeasures are:
1) Increasing the distance between the parts and wires that couple,
2) Shielding.

Of course, this answer is just as vague and general as your question.

[Chuck H]

It's not likely to be capacitive coupling UNLESS you actually have signal leads for multiple channels sharing a multi conductor shielded lead AND the unused channels are not grounded when not in use (rare but not unheard of). This IMHE. More likely something about how the circuit is wired, poorly performing decoupling caps on nodes shared by multiple channels, a less than ideal ground or even a funky tube. I've built quite a few custom design amps and had to trouble shoot problems like this many times. It's never been capacitive coupling.

We need a schematic to do better than this.

[GainFreak]

Moving/touching wires doesn't change anything. Tried to decouple that stage separately without result. I'm waiting for some documentation on this amp and will let you know with more detail. It's a hand built PCB amp, the tubes' sockets are mounted separately and wires run to the PCB.

[GainFreak]

Attached is the schematic and layout of the amp. Only when V3B pin 7 is grounded signal leak is gone (all Gain And Volume controls down).

MC II.pdf

[Chuck H]

Check relay K4 (and K5 which is switched via K4). Check C30. Possibly parallel a known good capacitor at least 10uf and of the same voltage rating and see if that helps. And check for good continuity and contact at the preamp ground via the board standoff.

[Jazz P Bass]

If everything on K4 is working properly, my only thought is the signal is coming in on C3 (V1A)

[trobbins]

Perhaps to further assist faultfinding, short the wiper at VR2 and at VR6 to its local gnd, depending on channel/relay operation, to see if you get the same leakage level for either channel setting.

[Pedro Vecino]

I see that the clean channel muting is done on its volume control. You can try muting the signal right at the relay input and see what happens.