weird burst of noise in the presence pot

[charvelguy]

I was cleaning out a Marshall's pots the other day and I noticed on the presence control I get this weird hum at the top end of the taper, between like 8-9, then when it's turned wide open it disappears. It's a very small window of noise and it can be avoided but I've never had a pot do this on an amp that I've noticed before..any ideas what it is? I don't believe it's a preamp tube.

[DonP]

Re: weird burst of noise in the presence pot

If the pot opens up (has dirt that causes zero resistance) this will "open up" the feedback loop, letting your Marshall run full out. The feedback loop that the presence control is in reduces the gain of the amp. The presence control "removes" the negative highs letting the highs have more gain (and more presence).

Sounds like you might be due for a new pot.

[charvelguy]

Re: weird burst of noise in the presence pot

Unless I'm misunderstanding you...I guess I didn't describe it well enough...it doesn't go full hot n sizzling wide open. It still has tonal taper throughout the spectrum from 0-10. It's just when it hits about 8 or 9 it gets this obvious unusable signal buzz or hum and then it disappears again at 10. So I can run it at zero thru 8 or 10, but not at 9 w/o a bunch of noise. The rest of the spectrum is fine. It's like a section of the pot is hot to RF or like the ground drops out. Weird.

Unless you are suggesting that when it hits at the open space on 9, the amp is actually running flat out open in the feedback loop causing more noise..but if that is the case, then why doesn't do the same when it is in wide full open position on 10? Should it not repeat the same with the same noise?

I appreciate your input.

[charvelguy]

Re: weird burst of noise in the presence pot

well I replaced the output tubes and the problem seems to have disappeared at all volumes including full out.

[DonP]

Re: weird burst of noise in the presence pot

Very strange. I still think your pot was openning up at 9, increasing the gain = increasing noise.

At the full 10 position, the "negative feedback highs" are sent to ground. Turning it to zero actually opens up the load to a max of 5K (total resistance of the pot). With the pot out of circuit, the feedback loop goes from 5K to infinite resistance.

I found out about this stuff when I was trying to design a Marshall with switchable tone and presence sections. I used a relay to switch between two presence controls, and every time you switched pots, there was a big increase in sound for a second (during the switching).

[charvelguy]

Re: weird burst of noise in the presence pot

interesting to know. Well I did shoot some tuner cleaner in there before all this took place. I worked the pots for a minute or so and then plugged it in. It could be it was still in need of dissipating some cleaner or displacing some internal dirt.