Low Power Tweed Twin DC on input jack

[tdlunsfo]

I got a strange one. I have two tweed twins sitting here, both 5E8A's and stock. One of them I rebuilt with all new caps but it has 5-600mV DC on the input jack of the bright channel and 60mV on the Normal channel and you can hear it on the guitar volume pot as well. I plug into the other twin with the same guitar and cable and no DC and no scratchy pots. Pulling tubes doesn't help and in fact makes it worse. Jacks have been cleaned but have the original 1M grid resistors. Checking resistance on both amps between jack tip and sleeve yields similar results between amps. Thoughts?

[Helmholtz]

Is the DCV positive or negative wrt ground?

If positive I suspect a conductive board.

One solution could be to add input coupling caps (e.g. 22nF)..

[tdlunsfo]

Positive DC. I've thought about just adding a .1uF but it bugs me that this issue exists on such a nice sounding amp.

[Helmholtz]

What is the voltage at the shared 470R cathode resistor?

[pdf64]

Pulling tubes doesn't help and in fact makes it worse

So with no valves fitted to input stage sockets, the voltage at the jack and grid terminal increases?

If so, that would indicate that the sockets are bad.

[Enzo]

If positive I suspect a conductive board.

If pulling the preamp tube does not remove the voltage, then I'd have to agree with Helms.

[tdlunsfo]

If bad socket then I should be able to pull the tube and see some resistance between pins correct? If conductive board I think we are looking at adding a cap on the inputs and moving on as the amp sounds pretty darn good otherwise.

[pdf64]

…
If positive I suspect a conductive board.
...

But with that model, there’s no input mixer resistors, I don’t think the grid circuit has any connection to the board, rather it’s a straight wire from jack tip to grid socket terminal.

https://el34world.com/charts/Schemat...5e8a_schem.pdf

[tdlunsfo]

But with that model, there’s no input mixer resistors, I don’t think the grid circuit has any connection to the board, rather it’s a straight wire from jack tip to grid socket terminal.

https://el34world.com/charts/Schemat...5e8a_schem.pdf

Correct. No grid stoppers

[Enzo]

According to the layout, the wires from the jacks to the tubs go through holes in the eyelet board so they traverse it underneath. However unlikely, it is possible the waxy goo on the board could penetrate the cloth push back wire.

I guess the question is this: is the amp wired with push-back insulation?

[Helmholtz]

Please post the requested cathode voltage.

[tdlunsfo]

Ok some new developments. The top of these sockets are filthy and it was hard for me to see until I shined a light on them. I cleaned them the best I could but problem is still there. Next I started moving some of the lead dress on V1 and V2 and was able to eliminate any DC on the Normal channel and was able to reduce the DC to 100mV on the Bright channel. Still got some pot scratchiness on Bright channel but it's better. Moving both V2 grid pins and leads make the most difference in my readings. I may change out these wires (original cloth) and see what happens.

[loudthud]

Are you using 12AY7 tubes for both V1 and V2 ? Check the Plate Voltages. Grid current frequently occurs when the Plate Voltage is too low. Also, check and report the Heater Voltage.

[pdf64]

… it is possible the waxy goo on the board could penetrate the cloth push back wire.
…

Yes, I’ve had that absolutely weird issue in a Sound City amp, so it’s another possibility here.

Are you using 12AY7 tubes for both V1 and V2 ? Check the Plate Voltages. Grid current frequently occurs when the Plate Voltage is too low. ..

Bear in mind that the issue occurs without valves in place.

[tdlunsfo]

Please post the requested cathode voltage.

1.71v