Presence Pot Differences

[plexi50]

What is the difference between these two different presence pot wiring pic's and is there one which is better than the other?

[jojokeo]

They both are very similar to each other doing the same thing electronically but w/ a different value pot and and added resistor component for the other pic. All it's doing is creating a voltage divider from the output tranny's tap and then using a higher value pot. Two different ways to do the same thing w/ different value components.

[sluckey]

The first one has no dc current flowing thru the pot, so no scratchiness.

[jojokeo]

Since the caps both going to ground (and w/out the schematic) how does that perform that job?

[FYL]

X goes directly to ground on the JTM50, with the cap connected between W and X but bypassed by the pot.

[sluckey]

Since the caps both going to ground (and w/out the schematic) how does that perform that job?

Well, in the top layout the cap is clearly in SERIES with the pot. Ain't no dc gonna flow thru that pot. Look at the schematics and you'll see what I'm talking about.

[plexi50]

The reason i asked this question is because i have finished building the 2204/Plexi and the amp sounds utterly fantastic but at almost full volume i start to get a little squeel on the 2204 channel. The Plexi channel is not effected. No grid cut off. If i roll back the presence just a bit the squeel goes away. So i was starting to question the presence pot and the way it was wired. I have never had any issues with presence pots before and this isnt one now. Just made me question it. I changed the bassman part to a plexi because i had that extra pot just taking up space and decided to go that route. I also installed a small rotary switch that changes the slope frequency from 100k-50k-35k. Nice touch. Theres something really wicked in the harmonics and tone of this build. Such fullness and depth yet easy to get clean and shimmery. Wavy like the ocean at certain volume setting. Anyway im wavying goodnight. I got to sleep /

[bluesbear]

It sounds like you may have some parasitic oscillation somewhere. It's probably not caused by the presence but cutting the higher frequencies makes it go away. I'd get it to where it's squealing and use the old chop stick to move the wires around and see if it goes away. Try tapping the preamp tubes and see if it affects the noise. Also, cold solder joints could cause it. Or... you could just back it off till it goes away, if the amp is for you and not a customer.
Good luck. These aren't always easy to find.
Dave