DC on input jacks?

[drewl]

Got a Weber style Fender 5E3 clone with around .6vdc on the end of a plugged in guitar cord causing the guitar pots to scratch.
The voltage is lower when loaded down/plugged into a guitar.
It's on both channels and all four inputs.

Each V1 cathode (pins 3 and 8) are grounded to the input jack ground for it's channel.
Think that's causing it?

I'm going to move the cathode grounds to another spot and try cleaning the chassis where the input jacks make contact.

Any other ideas what might be causing it?

[g1]

Have you tried another tube in V1 position? Sometimes a bad tube will have DC on the grids.

[Tom Phillips]

Got a Weber style Fender 5E3 clone with around .6vdc on the end of a plugged in guitar cord...Each V1 cathode (pins 3 and 8) are grounded to the input jack ground for it's channel.
Think that's causing it?...

No. That grounding method itself should not cause your problem.
Edit: Oops...See the next two posts in this thread.

[Leo_Gnardo]

Each V1 cathode (pins 3 and 8) are grounded to the input jack ground for it's channel.
Think that's causing it?

Grounded directly to the input jack ground? No cathode resistors? If that's the case then deafinitely you will have DC offset at the grids. And that will make your guitar controls scratchy.

Grounded cathode pre circuits usually have a cap to block DC at the input, say 0.01 uF 200V film. And a grid-leak resistor typically 220K to 2M2 on the grid side of that cap. Then they sound terrific & make a bit of their own compression when you smack 'em hard with a big guitar chord.

[Tom Phillips]

...Each V1 cathode (pins 3 and 8) are grounded to the input jack ground for it's channel...

Grounded directly to the input jack ground? No cathode resistors?.

Wow. I missed that and was thinking of the cathode resistor ground. If this is a 5E3 incorrectly modified to grid leak bias configuration or a mistake in the intended wiring that could explain the issue.

[drewl]

There are the typical cathode resistor and cap on each cathode.
THEY are grounded to the input jack ground lugs.

Sorry.
But like I said it's a 5E3 circuit, so of course it has 820ohm/22uf on each cathode and they are grounded to the input jack ground lugs.

of course I tried other 12AX7's - with no change.

[Enzo]

OK, is it .6v on the jack? Or is it -.6v?

[Leo_Gnardo]

There are the typical cathode resistor and cap on each cathode.
THEY are grounded to the input jack ground lugs.

Despite "standard" 5E3 schemo I had to make sure you weren't trying an alternate pre. Had me going there for a minute.

(Out of blunders - even imaginary ones - sometimes come great sounds. I'm going to have to try a grounded-cathode pre in a 5E3 now...)

Now let's look for possibly reversed electrolytic caps in those cathode bypasses? And a meter measure of cathode-to-ground just to make triple-sure there's nothing unusual going on. After that, I'm out of ideas for the night.

- - - -

Hey, Norristown, isn't that where Willie "that's where the money is" Sutton camped out for a couple years?

[Enzo]

Isn't that where Valley Forge Beer was from?

[Mick Bailey]

Your circuit board may be leaking DC due to absorbing atmospheric moisture; check for DC on the amp volume pots (turned right up) - do these scratch as well?

[km6xz]

Is the input shunt resistor in good condition? If I remember corrected it would have 1meg shunting to ground at the input. If that was open or drifted high, I would expect an unterminated cable plugged in to have a small charge.

[g1]

There is some speculation here about different circuits, just so everyone is on the same page, is this the correct schematic?

[km6xz]

The cable, unterminated is a capacitor that is about 40 pfd per foot, not a lot but across 1meg it can hold a charge seen on a high Z DMM or VTVM

[drewl]

That's the correct schem g-one
Enzo- there is a negative(- .7v )on the input jack with the amp in standby, with the amp on the voltage swings to a positive + .6v

The cathode cap is installed correctly.
There is a negligible dc voltage on the pots and they do not crackle.
There is no dc voltage after the plate coupling caps.

Any other ideas?

[drewl]

Back from vacation, and back to trying to figure this annoying thing out.

No other ideas eh?

I'll start by rewiring the input jacks and cathode circuit.