DC at the input, scratchy guitar volume pot

[Silvio55]

Well, after reversing the bias cap my amp worked, but I´m having this little problem now, my guitar volume pot is scratchy with this amp, so I looked for DC at the input of the amp and it was there, -0.8VDC in stand by and -0.3VDC with the amp on. I tried changing tubes but the problem is still there. What can be the problem? What should I check first?

[R.G.]

what guitar? what amp? link to amp schematic? does the guitar have onboard electronics?

[Silvio55]

what guitar? what amp? link to amp schematic? does the guitar have onboard electronics?

Guitar is whatever guitar I tried, all with passive circuits, and all worked fine with other amps, no scratchy noises, the amp is a 50 Watt Plexi 1987 clone, but without 68k gridstoppers at the input, jack connects straight to V1 grid through a shielded cable, and the ground point is right at the input jack (I used this method as recomended by Paul Ruby and worked fine with my other amp, no noises, no scratchy pots) here's the schem of the Marshall 1987:

The only other mod is the lar mar PPIMV, otherwise is just like this schem.
EDIT: I also conected the standby switch after the rectifiers, so the bias voltage is present even with the amp on stand by, unlike this schem

[R.G.]

That helps. Check and see if your 1M resistor to ground at the amp input jack is open.

[Silvio55]

That helps. Check and see if your 1M resistor to ground at the amp input jack is open.

I checked that too, and the 1M resistor is fine

[R.G.]

Just so I'm clear - the amp has 0.6 to 0.8V DC on its input jack just sitting there? And the 1M pulldown on the input is OK?

If thats true, try swapping in another tube for the input.

A voltage of 0.8V across 1M is a current of 0.8V/1M = 800uA. It's coming from somewhere, and if there is no other connection to that resistor, it's coming from the tube. Er - you *have* checked for undesired other stuff connected to the 1M, not just the tube and the input jack, right?

[Silvio55]

Just so I'm clear - the amp has 0.6 to 0.8V DC on its input jack just sitting there? And the 1M pulldown on the input is OK?

If thats true, try swapping in another tube for the input.

A voltage of 0.8V across 1M is a current of 0.8V/1M = 800uA. It's coming from somewhere, and if there is no other connection to that resistor, it's coming from the tube. Er - you *have* checked for undesired other stuff connected to the 1M, not just the tube and the input jack, right?

Yes, I have 0.8VDC at the input with the pulldown resistor ok, I did try swapping tubes and the problem is still there, and I did check for other conections to the resistor and couldn´t find anything wrong

[R.G.]

Hmmm... next I'd check for a shorted/leaky cathode cap on the input tube(s). Pull the cathode caps of temporarily on the input stage. If problem goes away, it was a bum cathode cap. If this doesn't help, I'm down to looking for tagboard leakage.

Or maybe it's haunted.

[nickb]

Just so as there's no confusion I know RG meant to say 800pA (0.8uA) That's a pretty small current.

Edit: See later - should have said 800nA.

[eschertron]

Just to add to the confusion, 0.8uA = 800nA

[nickb]

Just to add to the confusion, 0.8uA = 800nA

Indeed it is!

[Silvio55]

Hmmm... next I'd check for a shorted/leaky cathode cap on the input tube(s). Pull the cathode caps of temporarily on the input stage. If problem goes away, it was a bum cathode cap. If this doesn't help, I'm down to looking for tagboard leakage.

Or maybe it's haunted.

Ok! I will try that, i don't know if it helps, but checked for DC at the input without the tubes, and then I got 0.0VDC

[g1]

without 68k gridstoppers at the input, jack connects straight to V1 grid through a shielded cable, and the ground point is right at the input jack (I used this method as recomended by Paul Ruby and worked fine with my other amp, no noises, no scratchy pots)

Do you have that other amp, if so try that exact tube in this amp. Perhaps with the lack of gridstopper it is extra finicky about the V1 tube and maybe you just got lucky in the other amp. You might also want to see whether a smaller gridstopper (10K or so) cures the problem.

[Silvio55]

Do you have that other amp, if so try that exact tube in this amp. Perhaps with the lack of gridstopper it is extra finicky about the V1 tube and maybe you just got lucky in the other amp. You might also want to see whether a smaller gridstopper (10K or so) cures the problem.

I´ve tried a lot of different tubes on the other amp, never had a problem, and I tried on this amps every tube I have in hand and always got the same readings.

[markr14850]

What are your supply, plate, and cathode voltages for that stage?

Do you have a scope? Check for oscillation.