Scratchy Treble/Bass Pots Question

[bbaug14]

I have a Plexi build where a rotary switch is wired to go between lead and bass spec (split/shared, V2a bypass, tone stack). On the bass setting everything is fine. When on the lead setting, I get scratch treble and bass pots only. All the other pots are fine with no noise. I tried swapping the tone stack cap I switch in (270p added in parallel with 270p), as I just figured it was a leaky cap, but it made no change. I'm going to get it on the bench tomorrow and was wondering if anyone has any suggestions? Since it works fine in the bass setting, only the tone stack cap/resistor, V2a bypass cap, and the split 2.7K/0.68u change. I tried both hi/lo inputs in lead setting and the issue is still there, telling me I can rule out the 2.7/0.68u, not that I thought it could be this anyway. I also don't see how it could be the V2a bypass. Anyway, if anyone had some thoughts, I'm all ears. Maybe lead dress? Both the resistor and cap are on the rotary, so I don't think lead dress can be any issue.

[sliberty]

DC on the pots? You can measure to see if this is the case. A schematic of the mod would help.

[Smokebreak]

Do you have plastic knobs on the pots yet?

[bbaug14]

Do you have plastic knobs on the pots yet?

Yes I do. I dont have a schematic of the rotary switch, but I could post a picture I suppose.

[bbaug14]

This is the switch.

[bbaug14]

What doesn't make sense to me is it only happens when I switch to the lead setting. In the bass swtting all is good, so thst means all the couplers are fine and the tone stack is good without the added 250pF. If its DC then the only thing that could be causing that is the tone stack cap I switch in. I've already changed that once but I suppose I could have had two bad/leaky caps in a row. Could a bad CC resistor do this?

Any suggestions for easily checking out the 2 250pF tone caps I have for leakage?

[bbaug14]

So I replaced both tone stack caps today with brand new ones. One on the switch and one on the board. Issue is still present. No scratchy pots in bass and scratch treble/bass in lead. I would really appreciate any advice you might have here. I'm at a loss as to what could be causing this only in the lead setting. Thanks.

[Structo]

What amp is this?

Do you have a schematic?

[bbaug14]

This is a JMP 50W built to 68 spec with the switch outlined above.

On thing I thought of is maybe the 2.7k/0.68u arrangement wire is to close to the plate wire on V1 but I wouldn't think the result would be scratchy pots.

Here is a chassis photo.

[bbaug14]

This is a JMP 50W built to 68 spec with the switch outlined above.

On thing I thought of is maybe the 2.7k/0.68u arrangement wire is to close to the plate wire on V1 but I wouldn't think the result would be scratchy pots.

Here is a chassis photo.

[bbaug14]

Pic

[Structo]

Have you tried with the rotary switch bypassed?

[bbaug14]

Well, no. I can put it on the bass setting and there is zero issue. Quiet as a church mouse through the sweep of the BMT pots. That tells me that the switch is working as it should and there is something in the lead setting causing DC to get on the pots. I've swapped the two tone stack caps thinking the bass might have been leaking a little and the lead cap switched in made it worse, but that wasn't it. The only other caps switched in lead mode are the SOZO 0.68u cathode bypass caps. I wouldn't think it would be that.

[bbaug14]

Just for completeness, I tried lifting the 0.68u bypass cap on V2a out of the circuit to see if that was the culprit. Nope. I'm really at a loss for what to try next. Since the BMT pots are all scratchy, and only in the lead setting, I would think it would have to be a cap that's leaky only in that setting. However, I'm not seeing this to be the case. I wouldn't think lead dress or wire routing would cause an issue like this either, especially since the Bass setting is dead quite. I'm stumped. Appreciate any advice. Thanks.

[tubeswell]

Could be insufficient filtering in the pre-amp supply node. (Are you using 8uf or 10uF filtering on the pre-amp node by any chance?)

If there is not enough filtering, then the V1 stages being supplied from the pre-amp node can start to self-oscillate (LFO) through the power rail. This causes DC to appear on tone stack pots etc that are supplied from that pre-amp node. (This can easily be confirmed if when you test the leakiness of the tone stack caps for DC, do you see varying +ve /-ve DC on the AC-side of the caps?)

One cure is to increase the pre-amp R-C node's filter capacitance.

Another 'fix' is to add more AC-coupling - to 'break' the DC pathways (e.g.; between the stage that is driving the tone stack and the tone stack) I've fixed this kind of self-oscillation this way before.

Not saying that this is your problem - trying to diagnose amp problems through a website (without having the thing on the bench in front of me) is part prior knowledge and part hunches.