Noisy presence pot

[EFK]

I'm wrapping up a build and I have the amp pretty much silent except the darned presence pot - it's scratchy and I can't shut it up! It is the only scratchy pot in the amp; the pot case has good continuity to the chassis and no matter where I run the ground wire (rigged like a Marshall circuit I guess - line in from NFB resistor on lug one, .1uF cap on lug 2 which grounds to lug three which has it's own ground wire via mostly star grounding in the amp). No matter where I run the ground wire, the pot is still scratchy. Not excessively loud scratchy, but it's scratchy and noticeable. Doesn't matter whether I use a plastic or metal knob either.

Any ideas? Thanks!

[Robert M. Martinelli]

Hi EFK,
the first thing I think about when I hear complaints about a scratchy pot ( apart from a defective pot ) is that some DC could be present on the pot's lugs. Should you find DC there, I'd advise you to check for a leaky capacitor in the NFB loop signal path.

Hope this helps

Best regards

Bob

[52 Bill]

Like Bob sez, there's probably DC on the pot.

Rewire so that the cap is in series with the pot and have the pot wire directly to ground.

[EFK]

Yep - there is DC on the pot, and I can see where it's coming from, but I can't figure out why. I'm using pretty much a straight JTM45 ltpi and presence circuit. I'm getting 22/23 volts on the intake of the pot and I'm using the same wiring as per the attached layout (taken from Weber's kit). Looking at this layout, what is keeping any DC off the 10K tail resistor from hitting the presence pot? That's where I think it's coming from. I don;t see how the .1 uF cap spanning the 1M and 10K resistors is having any effect on the jumper over to the junction of the nfb resistor. I know the cap is good, the cap on the pot is good, grounds are good, pot itself is good. ???

52Bill I'm not sure how you're saying to rewire?

[Chuck H]

That presence circuit is often scratchy sounding. Vinage Marshalls and modern clones alike suffer the same malady. But it only happens when the pot is being adjusted. With the pot stationary there is no detriment to the tone with this circuit. So, unless you spend a lot of time rotating the presence control while playing, I say just don't worry about it.

[Robert M. Martinelli]

EFK,
given the schematic/layout you used, DC can come either from the preceding stage ( cathode follower ) through a leaky cap in the tone stack, or from the LTPI's cathode network.

If the tone stack works properly ( no scratchy pots ), then I'd look at the LTPI. The "tail" resistor being 10K, a tail voltage of around 25 V seems in the ballpark, so I'd bet the DC you're measuring on the presence pot is coming from there.

Bill's advice of wiring the NFB cap in series and the pot to GND could cure the issue.

Hope this helps

Best regards

Bob

Edit: Just seen Chuck's post, and, yes, I agree that if you don't spend a lot of time adjusting the presence control, you might live with it just as well

[Chuck H]

Heres your other options. Option 1) is how I do it and for whatever reason it doesn't scratch much (or not at all) on my amps. Option 2) is a newer Marshall circuit. It will not scratch, ever. But it also won't allow you to turn the presence control to 0 like the one you have now or figure1.

Chuck

[EFK]

I love visuals! Thank you guys. I would say it generally wouldn't bother me except I've got the amp so quiet in all other ways that it's that one little thing that's going to drive me nuts. Chuck I might try it both ways - your method is the easiest as I just need to rearrange the wires. I'll have to look and see if I have a 25K pot to try the newer method. Thanks again!