Stumped by a scratchy pot

[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!

[km6xz]

Can you post a schematic? Is the wiper connected directly to the grid with no coupling cap and grid resistor? What stage is it and what is supplying its signal. If the pot wiper is connected to the grid directly I would expect noise since there is a grid leak path to ground that is varying in resistance depending on movement of the pot.

[Leo_Gnardo]

Gaz I just put a suggestion up on TAG where you asked the same question. If it works do everyone a flavor and drag it over here, OK?

[J M Fahey]

You should scope the tube plate for confirmation, but I found that sometimes an unstable amp, on just the edge of oscillation, gets in/out of it , of course we can't hear it by itself (very high frequency) but the start/stop sounds like scratching.

That said, I also found that often the added scope cable capacitance when connected to the plate, which may be as large as 100 or 200 pF, is enough to kill oscillation, so when you scope it, there's none (and no scratch) ... quite frustrating.

Just for confirmation, add 100pF to ground either at the grid, the plate, or, worst case, both.

If "scratching" stops, now you know.

[Gaz]

Thanks for the suggestions. Here is a drawing I did of the preamp.



I took this amp to a friend's house, and sure enough it did not make the scratchy/static noise... but it still does it at my place, and no other amps do. Could something with the mains be exacerbating the issue? The noise does stop when I touch the chassis. FWIW, I have confirmed the mains wiring is valid with one of those IEC checker things.

There is another issue however that did persist at my friend's. When the amp's taken off standby the the input lug of the pot shoots up to about 1.2vdc then slowly fades down in about 10-20 seconds. During that time I can actually read DC, and there is a much louder typical WHOOSH sound when rotating the pot. What could cause that? I am not sure if it's related to the initial static problem.

The scratchiness persists even when the input lug is disconnected (meaning only the wiper and ground lugs are connected), but the whooshing DC spike coming off standby stops, and the normal grid charge quickly drops to 0vdc).

I've tried doing low-passing at the 1st and 2nd stage plates as well as at the grid as JM suggested, but no help. How could I go about scoping for this kind of thing?

Thank you.

[Enzo]

If I had to guess, I'd say that 470pf cap from grid to cathode is putting a charging current through the grid resistors.

I'd still be thinking ther is some sort of RF thing happening in your house, especially if the amp does not do the thing elsewhere.

[Gaz]

If I had to guess, I'd say that 470pf cap from grid to cathode is putting a charging current through the grid resistors.

I'd still be thinking ther is some sort of RF thing happening in your house, especially if the amp does not do the thing elsewhere.

Thanks, Enzo. Just tried removing the 470p cap, but no change. This is also happening without the guitar cable in, so the first grid is shorted to ground. Again, what I'm seeing is the grid right after the pot sitting at about .7vdc with the gain control all the way up on standby (I have a 220K across the SB switch by the way that leaks just a little HT voltage), and it goes down to zero if I turn the pot down grounding the grid. From what I understand the grid charge is normal, and I'm seeing it on the other grids as well. The weird thing is that when I flip off standby the grid voltage 'spikes' up to 1.2vdc then settles down. In the interim there is a loud whooshing sound from the DC on the pot that does away once the voltage bleeds down to under 20mV. I can't say I've ever seen this before.


With the original static-like noise issue, I have to agree it's a fluke thing with the house... It just went away completely for no reason at all. It seems random for sure.

I'm also not sure about scoping for any instability, but I scoped the plate after the pot, and output of the amp, and the 1k sine wave is very smooth with no irregularities. I scoped without signal, and the line is totally flat... not sure if I'm doing that right or not.

[hylaphone]

Does this chassis have a shielded cover when installed in the cabinet?
I've had similar problems with an open chassis on the bench..

[Gaz]

Does this chassis have a shielded cover when installed in the cabinet?
I've had similar problems with an open chassis on the bench..

Yes, the static persisted, but as I mentioned it seems to have disappeared for now and changes with location. I also noticed after posting another amp was doing it, so it kind of confirms it's not related to the amp itself.

Now as far as the DC coming off standby, I checked another amp and it does the same thing as the voltage comes up. So now i feel crazy because I actually think this is normal behavior (or is that crazy?). I think the little DC spike coming off standby is exacerbated by the fact all those caps in the FAC have to charge up... that's my theory at least.

Additionally, I think I never noticed it before either because I'm not normally erratically turning the gain control as the amp comes off standby, which is what I was doing trying to solve the static issue.

I have a Twin Reverb that does the same thing when it comes off standby if I quickly turn the gain control... just a little whoosh whoosh. In both of these amps the standby is before the first filter cap.

Could someone confirm this little surge is normal behavior and possibly explain to me what's happening when the amp comes off standby and I read a little DC on the pot?

Thanks!