Marshall 2555x Presence control

[ziggy007]

Gday all,
I have a 2555x Silver jubilee (reissue) on bench. They use a C22k Pot for the presence control. Any idea why they would do that? Seems like a weird idea. I get what it does, not why? Surely you are just adjusting the NFB to a point (Between 1-10 on pot) you are happy with and away you go? Happy to have smarter minds than me correct/school me!
Cheers
Gav

[Enzo]

It is just a guitar amp. Like every knob on an amp, you turn it to the point it sounds best to you, and there you are.

[loudthud]

Link to schematic: https://el34world.com/charts/Schemat..._100w_2555.pdf

There is more than one circuit commonly used for Presence controls. In this amp, the tail current flows through R20, not through the Presence control like the 5F6A and 60's Marshalls. This makes the Presence control silent because there is no changing DC across the Presence cap, C14. The disadvantage is that C14 is never totally out of the circuit.

[ziggy007]

Yep cool. Thx Enzo. agree. Hence why they would do this confuses my simple brain. A 22k anti log pot is unobtanium in Australia. it’d be weeks for shipping from OS. I’m just going to put a 25 log in there wired backwards. Cheers

[J M Fahey]

C is the right taper for a smooth action; since itīs in the lower leg of NFB, an anti-log pot there is same as a log one in the upper half, both will match best what our ears hear.

BUT everybody has used a linear pot there since Day 1, and Iīm talking 50īs Fender Tweed amps on, so do the same.

A reverse wired Log pot will be most counter intuitive and baffling for a user, since raising Presence will *lower* it ... you donīt want that.

Just go linear and call it a day.

[Zozobra]

A reverse wired Log pot will be most counter intuitive and baffling for a user, since raising Presence will *lower* it ... you donīt want that.

Rename it muffle :lol: