Signal leaking through with volumes off

[J M Fahey]

These points are at 0V DC and can be grounded with a piece of wire.

* Node (junction of) C17-R35

* node C29-R36

does any of those kill signal feedthrough?

[Greg_L]

These points are at 0V DC and can be grounded with a piece of wire.

* Node (junction of) C17-R35

* node C29-R36

does any of those kill signal feedthrough?

Yes to both.
C17/R35 kills the leak from the normal channel.
C29/R36 kills the leakfrom the vibrato channel.

[Greg_L]

- unsufficient node D filtering: Check filter cap for C and ESR, scope voltage at node D for residual signal.

I don't currently have a way to check ESR, but I did not find any input signal riding on the D node with the scope.

Everything with the power nodes and their grounds seems to be okay.

[Helmholtz]

..but I did not find any input signal riding on the D node with the scope.

Good. Now please also scope the V2b cathode (pin8) for any mV signal - still with V1b grid grounded.

Do you see the leakage signal at the PI input (scoped at the R35/R36/C14 node)?

[Greg_L]

Good. Now please also scope the V2b cathode (pin8) for any mV signal - still with V1b grid grounded.

Do you see the leakage signal at the PI input (scoped at the R35/R36/C14 node)?

I'm sorry I don't fully understand.

Am I looking for the leakage at the PI input while also grounding the V1b grid and scoping the V2b cathode?

[Helmholtz]

I'm sorry I don't fully understand.

Am I looking for the leakage at the PI input while also grounding the V1b grid and scoping the V2b cathode?

No, these are 2 independent measurements. You may start with scoping the PI input as described. Actually it shouldn't be necessary to ground the V1b grid as you reported no difference with vol pot at zero. Just looking for leakage at different circuit points (of course with some input signal and both vol. pots at zero) in order to identify the leakage path ("leakage tracing").

[Greg_L]

No, these are 2 independent measurements. You may start with scoping the PI input as described. Actually it shouldn't be necessary to ground the V1b grid as you reported no difference with vol pot at zero. Just looking for leakage at different circuit points (of course with some input signal and both vol. pots at zero) in order to identify the leakage path.

Okay understood. I will check that later and report back. Thank you.

And yes, there is leakage at the PI input node with nothing else grounded and the vol pot at 0. It happens with both channels.

[Helmholtz]

And yes, there is leakage at the PI input node with nothing else grounded and the vol pot at 0. It happens with both channels.

As there is leakage at the PI input, you should also see it at the V1b plate. Which is larger?

[Greg_L]

As there is leakage at the PI input, you should also see it at the V1b plate. Which is larger?

I will check and let you know, thank you.

[Greg_L]

As there is leakage at the PI input, you should also see it at the V1b plate. Which is larger?

Okay I ran your tests and here's what we got:

V1b grid grounded, no signal at V2b cathode. Clean as a whistle.

The leakage at V1b plate is about 75mv pk-pk. The leakage at the PI input is roughly half at about 40 mv pk-pk.

[Helmholtz]

Okay I ran your tests and here's what we got:

V1b grid grounded, no signal at V2b cathode. Clean as a whistle.

The leakage at V1b plate is about 75mv pk-pk. The leakage at the PI input is roughly half at about 40 mv pk-pk.

Thanks. The voltage values tell me that PI input leakage originates from the V1b plate and gets voltage-divided before it reaches the PI.

And what about V1b cathode with V1b grid grounded?

[Greg_L]

Thanks. And what about V1b cathode with V1b grid grounded?

From memory it was also clean, but I'll double check that later. I gotta step out for the day.

[Helmholtz]

The "leakage" signal at V1b signal would be very small, say between 1mV and 2mV. With the grid grounded, the tube will operate as grounded/common grid amplifier and any cathode signal will appear amplified by a gain factor of around 60 at the plate.
In other words, a little over 1mV at the cathode suffices to produce 75mV at the plate.

[Greg_L]

The "leakage" signal at V1b signal would be very small, say between 1mV and 2mV. With the grid grounded, the tube will operate as grounded/common grid amplifier and any cathode signal will appear amplified by a gain factor of around 60 at the plate.
In other words, a little over 1mV at the cathode suffices to produce 75mV at the plate.

With V1b grid grounded I'm seeing nothing on the V1b cathode. Clean.

[Helmholtz]

With V1b grid grounded I'm seeing nothing on the V1b cathode. Clean.

If my hypothesis of a grounding issue is correct, the signal leakage showing at the V1b cathode would depend on where you connect the ground lead of your probe. A good grounding point should be the negative lead/terminal of C20.