Presence Pot

[C Moore]

http://el34world.com/charts/Schematics/ ... w_1959.pdf

For this Super Lead schem.
The presence pot.
I understand (I Think) how it works.....separating certain, higher frequencies from the NFB, so those AC signals do or do not get into the NFB circuit.
Is there normally DC on that pot.?

This Marshall has a presence pot that is noisy through its entire rotation.
It has (from ground to wiper) about 25mVDC on Zero and increases to about 15VDC with the pot on Ten.
Is this normal...where does the VDC come from.?
Thank You

[Andy Le Blanc]

look at the phase invertor, the pres. is between the ground and cathode

[roberto]

Yes, it's normal.

The DC comes from the current flowing inside the two triodes of the PI.
That's why it's scratchy.

[C Moore]

So that 15VDC is idle (cathode/plate) current, across the 5k pot.?
I guess I do not remember Presence Pots being this noisy.?

[roberto]

That's because most of the presence pots use a different schematic.
Basically a 4k7 resistor to ground in parallel with the presence pot+cap.

This way you have no DC across the pot, only AC, and the pot doesn't crackle.

[C Moore]

That's because most of the presence pots use a different schematic.
Basically a 4k7 resistor to ground in parallel with the presence pot+cap.

This way you have no DC across the pot, only AC, and the pot doesn't crackle.

Yeah.....looking at some other schems now...I see what you are saying.
Thank You

[roberto]

Perfect!
You will notice the same on depth pots if you don't have a dc blocking cap in series with the NFB wire (positive facing the PI side).

I personally prefer not to have that cap, as it sounds IMHO better without.