Scratchy presence control

[Axtman]

I am currently repairing a Marshall 1987X 50 watt head. The Presence control is really scratchy. I cleaned the pot but still have the noise. I checked for voltages and I have about 14 vdc on one leg of the pot.

This voltage comes from VL2 connection as seen on the 2nd page of the schematic lower left hand corner. Ideas where this voltage comes from and how to block it on the pot?



Schematic here: Marshall-Plexi-Superlead-Reissue-50W-1987X-Schematic.pdf

[Helmholtz]

That DCV across the presence pot is normal with these vintage type amps.
Reason is that the presence pot is part of the cathode tail of the PI with around 3mA DC running through it.
Scratchy presence pot is a known consequence of that wiring.

[Axtman]

Thanks Helmholtz!

[dai h.]

(not sure if I'm mis-remembering, but) I've used one of those 2W sealed Allen-Bradley pots there in a 'guinea pig' Marshall and it seemed less scratchy. (Has anyone else ever tried that and can confirm?)

[Mick Bailey]

Some pots are less scratchy with DC present - it depends on the wiper type and track material. Some pots have a carbon brush rather than metal wiper or fingers and this seems to minimise noise (though not eliminate it).

[dai h.]

Thanks. Yes, those A-B's have a little carbon block(?) inset in a big (brass?) wiper instead of a metal (or plastic) piece with fingers (it looks like a minature version of that found in wirewound rheostats). I tried to find some more info on possibly quieter pots such as wirewound (small enough to fit ones are available but apparently may not have a good lifetime in the number of rotations), cermet, and carbon film but not quite sure if they would be better options (latter seem hard to source).

[Chuck H]

Not sure if I'm out of my mind or about to misrepresent any circuit function, but...

I wire my presence controls in the standard DC on the pot manner used on vintage Marshalls, with one exception. Rather than having the cap connected between wiper and ground I have it connected from the feedback lug to the wiper. Now, a cap and resistor in series is the same circuit regardless of orientation. I understand this. And I know that there is DC on the wiper in both circuit incarnations but there seems to be a difference. I may get some corrections on my concept here. Fine. But none of my presence controls have ever been scratchy like the typical circuit and I never understood why. It's just the way I wired up the presence control in an early build and since it worked I've been doing it that way ever since.

EDIT: I'm not in one of my amps right now but if I recall I do invert the lug orientation for this.