Unusual scratchy gain control problem

[Gaz]

I just finished a build, and am having a an issue with a scratchy gain control.

There is no DC on the pot, and I've tried the following with no improvement:

- Changed 12AX7 whose grid is connected to pot
- Changed pot
- Changed ground wire from pot and tried different ground points
- Drastically altered lead dress
- Cleaned flux residue around tube socket

If I disconnect the input lug of the scratchy control there is still scratch with just the ground and wiper connected to the grid. That lead me to believe it was the tube itself or pot, but it's not.

The only thing that makes the noise go away is touching my hand to the chassis while rotating the control. What could this mean?

I thought maybe a grounding issue, but everything checks outs. I only have one ground point at the input jack, and it's a method I use often with no problems.

HELP!

[Stevem]

Does adding or removing a sheild over the tube change anything?

[Gaz]

No, unfortunately

I should also add that the topology is Input -> 1st Stage -> Scratchy gain control -> 2nd Stage, etc.

The amp otherwise works normally, and it's the only control that scratches.

[Leo_Gnardo]

Does this control have a metal shaft? Some such controls seem to develop static between shaft and bushing, resulting in noise. I use a tiny dab of what's referred to as "nut grease", a mix of graphite and Lubriplate grease usually used in guitar nut slots. The graphite is conductive enough to knock down the static, & scratch noise goes away. It's always been a common problem on 70's Ampegs, that's why I developed the solution. All you need is a fly speck dot of nut grease, applied at the spot where shaft enters bushing. I use a sharp pointed toothpick and cram it into the gap. No worries if it doesn't look "perfect", as long as you got some of the conductive goop into the gap. Besides it's going to be hidden behind a knob. You don't want to put in so much it will find its way into the pot and play hob with the innards.

[pdf64]

Have you scoped it for oscillation?
Good lead dress may only go so far; the fitting of grid stoppers to every stage is good practice, eg 10k to 22k for every common cathode 12AX7 stage.

[Stevem]

x2 with the grid stoppers!

[Structo]

So the caps test good that feed the pot?

[Gaz]

Thanks for all the suggestions! I've done some more snooping since posting.

I brought the amp to a friend's, and sure enough the noise went away, but is back again at my shop. Could anyone think of a reason this would be related the power in my home? I have an IEC checker, and have confirmed the mains wiring is correct. Other amps at home do not do this however, so something is making this particular amp susceptible.

Again, the noise stops when I'm touching the chassis (including via the guitar strings)

@ Leo. Unfortunately, it's not the pot, I've already changed it out. I've used this brand many times (Bourns) with no issue, but that is good suggestion nonetheless.

@ Steve. Yes, all stages have grid stop. I went up to 500K on the one connected to the wiper of noisy pot with no help. The grid wire itself is only 1", and I've tried replacing it with shielded as well. No help. Currently grid stop is 10K.

@ Structo. Yes caps are good, and no leakage. Again, even with the input lug of the Gain pot disconnected there is static noise.

@ pdf64. How/where would you try to scope in this type of situation?

One other odd thing that may be connected to this issue is that when the amp comes off standby there DC voltage on the input lug of the shoots up to about 1.2vdc then fades down after about 10-20 seconds. During this time the volume pot does the "whoosh, whoosh" sound of DC (which I can actually measure) that is different from the static noise. I cannot figure out why this is happening!

I attached a drawing of the preamp.

Thanks!

[JTU]

Have you tried the amp without the switch arrangement? Might be something accidentally touching or a faulty switch...

[Gaz]

Yes, I've bypassed the relays and FAC completely, so it basically just goes, plate, cap, pot, grid stopper, grid.

[Stevem]

If your input Jack is fully insulated from ground then in its hot and ground connection lead place a coupling cap of any value for now and see if the issue stops.

[pdf64]

I suggest scoping the 2nd stage plate and the speaker output.
Set the amp up so as to maximise the 'scratch', and under idle conditions check there's no free running oscillation at either point, over a range of volts/div and time/div scope settings.
See if anything becomes apparent when the scratchy vol is operated, again, over a range of scope settings.
Then see what test signals of various levels and frequencies look like.

[Structo]

When you take it off standby, are there filter caps after the switch?

If so, that is why the voltage jumps for a bit.

Do you have the open side of the chassis shielded?

9 out of 10 times when a new build has problems, it is a wiring error or bad ground or both.

[tubeswell]

Is there a high-res pic of the problem part of the circuit (so we can see for ourselves)?

[Gaz]

I suggest scoping the 2nd stage plate and the speaker output.
Set the amp up so as to maximise the 'scratch', and under idle conditions check there's no free running oscillation at either point, over a range of volts/div and time/div scope settings.
See if anything becomes apparent when the scratchy vol is operated, again, over a range of scope settings.
Then see what test signals of various levels and frequencies look like.

Thanks for the instruction. I did this and did not see anything strange at all. The scratching also went away in my shop the came back, and now another amp is doing the same thing. I have to assume for now it's something with the mains or RF at my house.