scratchy presence control 5F6A

[Randall]

I have a 5F6A Tweed Bassman clone on the bench. It's a nice build. It all pretty much works, except the presence contol is scratchy. I have 14vdc on the top of the control looking at the schematic. This seems reasonable given there is supposed to be about 32vdc at the junction of the four PI resistors. I would assume some of that voltage will appear on the other side of the 10K resistor that connects to the pot. But, DC voltage on pots make them scratchy. So which is right?

https://ampwares.com/schematics/bassman_5f6a.pdf

[Helmholtz]

With 32.5V at the junction of 4 resistors (please verify) and correct resistance values, the voltage on the presence control should be just below 10V (i.e. 30%).
14V would be too high.

And yes this wiring is prone to pot turning noise because of DC.
But with a good clean pot, results typically are acceptable.

Try Deoxit on the pot.

BTW, the grid voltages in the schematic are way too low, probably due to loading by the meter.
Voltages on both ends of each of the 1M resistors must be the same as there's no grid current.

[Randall]

DeOxit was the first thing I did.

Resistors are verified. I have 47vdc at the junction of the four resistors. Where is it coming from?

[Helmholtz]

High cathode voltage means increased tube current.

My first suspect would have been the 470R biasing resistor, but could be a high gm tube as well.
It's not necessarily a problem.
What are plate voltages?

I don't think pot noise would be much less with lower voltage.
Is the 0.1µ presence cap good? If leaky that could increase noise.

You could change the presence wiring like with later Marshalls, avoiding DC through the pot.

[Chuck H]

You could change the presence wiring like with later Marshalls, avoiding DC through the pot.

I've considered this for new builds and always skipped it. With the 25k pot circuit the presence is always on just a little and sometimes I actually do like it all the way off. I haven't had any trouble with scratchy pots but even if I did I wouldn't care because I'd know better than to adjust the presence control in the middle of a track on a recording!?! Otherwise who cares?

[Tom Phillips]

It is my feeling that the basic classic design of the Fender Presence circuit (Variable presence pot in parallel with the cap) is the problem. With that design there is always DC on the Pot and it will therefore always be scratchy. The fix is a small wiring mod putting the pot (wired as a rheostat) in series with the presence cap.

[The Dude]

There's no way to avoid the noise with this presence circuit. You'll either have to use a different circuit, or the customer will just have to deal with it as is. This isn't something wrong with that particular amp. It's a typical side effect of the circuit.

[pdf64]

Another potential cause of scratchy pots is oscillation.
The 5F6A doesn’t have grid stoppers, which makes oscillation more likely https://el34world.com/charts/Schemat...ssman_5f6a.pdf
Do any other controls affect the point on the pot that scratches?

47VDC there seems high, but may not be unreasonable, especially if the HT voltage is higher than the schematic nominal.

[Randall]

This amp is running a B+ of 494v.

[Leo_Gnardo]

Tom Phillips & The Dude have it right - in that circuit there's always some DC on the presence pot, and because of that you get a scratchy noise when you move it. What's printed on some very x-pensive boo-teek overdrive pedals near some control: "scratch OK." Either have to live with it, or alter the circuit.

[Randall]

Good to know. Don't fix it if it ain't broken.

[Tom Phillips]

Tom Phillips & The Dude have it right - in that circuit there's always some DC on the presence pot, and because of that you get a scratchy noise when you move it. What's printed on some very x-pensive boo-teek overdrive pedals near some control: "scratch OK." Either have to live with it, or alter the circuit.

This issue is another example resulting from individuals blindly cloning original circuits.