Dreaded Scratchy Pot Syndrome

[rockybottom16]

Hi All-
After replacing my strat's volume pot to no avail, I've realized that the Marsh 5E3 I built a couple months ago must have the scratchy pot syndrome. What's amazing is that I've been so taken with how great this thing sounds that it took me this long to start paying attention to that slight scratchy sound that comes from my speaker when I move the guitar volume between 10 and 7. Does this mean I've got DC leaking into my guitar? How does this happen and what can I do to fix it without altering the tone of my amp? I'm not an engineer and this was my first build, so please be kind.

I appreciate your help!

Ken

[Tremolux55]

If you have leaking DC then one of your coupling caps is bad.

Depending on your level of confidence / patience you can try a number of things, all lead to replacing a bad cap.

1. Lift the following end of each coupling cap (0.1 uf) and measure for DC coming through - usual caveats about voltages etc. You should be able to identify the culprit.

or

2. If you have a spare 0.1 uf cap replace one cap at a time until the bad one is found - ie take the first cap out and replace with the spare. If you still have scratchy pots use the cap you removed and replace the second and so on until you ahve fixed the situation.

Like you I'm no engineer and there are more qualified people that read these forums so they may have other / better ideas but this has worked for me in the past.

Good luck - this is part of the fun / addiction of building

[tubeswell]

Either DC on the pot, or, if you made a clumsy job of soldering, maybe leaving some flux or other crap on the wiper contacts at the 7-10 setting.

When I'm soldering pots I heat the lugs up gently and tin them with a bit of solder and do the same with the wires (altho' more generously) before soldering them. I lie the job so the wiper surface is horizontal, ensuring that the wet solder and any other surplus gook that happens to be on it doesn't end up inadvertently running into the wiper assembly while the solder is still hot, and I lie the tinned wire against the pot lug and heat gently just for long enough of the solder to flow and form a good joint, but not too long so as to damage the pot.

I keep the iron clean between solders straight after soldering each joint, by wiping the hot tip off on a wet sponge I have sitting in a little bit of water an old plastic ice-cream container on the bench nearby. I reckon having a clean iron tip solves 50% of the problem with bad joints.

[rockybottom16]

Thanks for your input, guys. Anyone else have any suggestions? I replaced the first 2 coupling caps, but still have the scratchy pot - no change. Should I try the next one(after the PI)? How would DC get past the two that I've already changed and into the inputs? I could use some help here guys - I know you're out there! I emailed Mike Marsh a few days ago but still haven't heard back. He sells a great kit and I look forward to ironing out this last minor issue.

[paddy]

Hi there,

I'd have a look at the input jack wiring and the wiring of the first tube.
Can you post any pics?

Cheers
Paddy

[rockybottom16]

Thanks for your advice, Paddy.
I'll have to look into how to post photos - never tried it before. M. Marsh emailed me back and said that if the first 2 coupling caps are good(which they are) then he doesn't think DC could be getting back to the input. He said he'd research it some more and get back to me. I'll check the wiring on the jacks and on the 12ay7 socket again, but the amp sounds great and everything functions as it should. Seems that if it were wired incorrectly it wouldn't work right. Also the scratchyness isn't noticable with the amp volume below around 3. Something I noticed yesterday is that when I run my digital reverb pedal between the guitar and the amp, the scratchyness is not there anymore. That's probably why it took me so long to notice it to begin with - I rarely play without the reverb.

[paddy]

No problems with the help mate,that's what the forum is all about.

I suspect that your pedal is blocking the dc which is otherwise reaching
your guitar volume pot.

It doesn't take a lot of dc on a pot to make it scratchy.Are your wires
from input jacks to the grid resistors shielded?(could be a lead dress issue)
Is it occuring on all inputs?
I would also check the values of the plate resistors on the first stage,because
if they go high,they can cause grid emission and a small amount of dc can find
it's way back to the guitar.
Some voltage readings to go with the build pics would also be good.

Cheers
Paddy

[pdf64]

Some tubes end up with a small dc voltage on their grid, causing this problem, just one of those things like some are microphonic, noisy, whatever. Try a different tube (12AY7). Tubes are on sockets for good reason, they should be the first thing you swap out when there'a a prob. Peter.

[stokes]

Caps is obviously not the problem since you have no caps connecting a dc source to your volume pot.A shielded input wire wouldnt help either as shielding isnt going to lessen the dc voltage any more than a straight wire would.I have seen this problem on many 12AY7 inputs,and havent been concerned enough to dig any deeper and always attributed it to a small dc voltage on the grid of the 12AY7,as pdf pointed out.DC voltage isnt going to backtrack to the input from further downstream,so it has to be a small amount of volts on the grid at the input.