Vox AC4TV scratchy noise from guitar controls

[tomtech]

Have a Vox AC4 that when my guitar is plugged in and I roll the vol control I get a scratchy noise from the speaker. If I flip the pickup switch I also get a staticy clicks out the speaker. Same guitar and cable to other amps (Fender Blues Deluxe for instance) are dead quiet when I do the same thing. There are no control setting on the amp that resolve the noise, save for vol off. Anyone experience this? Resolve it? Thanks.

[Helmholtz]

Please post schematic.

Do you measure any DCV at the amp's input with guitar connected?

[tomtech]

"Do you measure any DCV at the amp's input with guitar connected?"
I did not check for this. Can you explain a little as to what you think might be happening??

[tomtech]

Here is the schem, missed it first time.

[Helmholtz]

If there's DCV at the input, a DC current will flow through the guitar's controls. And that makes them scratchy when turned.

[tomtech]

http://el34world.com/charts/Schemati.../Vox_AC4TV.pdf

[tomtech]

Got it.

[Helmholtz]

If you find any DCV at the input, try a new 12AX7.

[vintagekiki]

http://dealers.korgusa.com/svcfiles/AC4TV8_SManual.pdf

1) Check the ground in the mains cable (wire E)

[tomtech]

Thanks for the service manual link VKK! On the testing of the dc on input, the amp is not in my possession now. I'll have to ask the owner if he has a VM and if he can test the input for DC. If he cant do this test it will need to wait till I have the amp back. I also thought the grounding might be a culprit, but AC GND lugs to chassis and spade terminals seemed OK from a visual. When I get the amp back I can give this more scrutiny. I also have other tubes I can try if nothing turn up on input test and GND check. Thanks for the help, I'll update this post when resolved with what I found.

[Jon Snell]

Unless your guy has active pickups and there is a problem with his guitar. I have had many with DC on the tone controls and that causes this sort of issue. Ask him to try a different amplifier to rule out the guitar.

[tomtech]

John Snell, Per my orig post: "Same guitar and cable to other amps (Fender Blues Deluxe for instance) are dead quiet when I do the same thing."
Happens using my strat (SSS) or ES335 (HB's).

[pdf64]

A possible cause of excessive grid current (which then manifests as grid voltage) can be a very low anode voltage. Perhaps due to a HT dropper or anode load resistor drifted up in value. May have been overstressed if the user tried substituting a different type, eg an ECC82, in that socket.

[Enzo]

Jon stated the exception, your test points back at DC on the input jack.

[Helmholtz]

A possible cause of excessive grid current (which then manifests as grid voltage) can be a very low anode voltage. Perhaps due to a HT dropper or anode load resistor drifted up in value. May have been overstressed if the user tried substituting a different type, eg an ECC82, in that socket.

Interesting and makes sense for "normal" grid leak current. What's your source?
Now it would be interesting to know if the grid voltage is negative or positive.
A negative grid voltage would mean "normal" grid leak, caused by electrons landing on the grid. This effect tends to self-stabilize as negative grid voltage lowers plate current and thus plate voltage increases.
A positive grid voltage would be caused by positive gas ions in a gassy tube. This effect has a tendency to runaway.

BTW, the scratchy pots effect should vanish when a pedal is used - if it's caused by DCV.