Low hum from BF Vibrolux

[Randall]

Amp came in for a re-cap job and a few other things. I did the re-cap, but it has a low hum with everything turned full down. Tubes are running at about 50% and 7mA apart.

What is throwing me is I have 48vdc on PI grid 1 and 52vdc on grid 2. Is this right? I never totally grasped the PI part of the circuit.

https://schematicheaven.net/fenderam...a964_schem.pdf

[g1]

Grids are tough to measure as you are introducing resistance with your meter. Should be close to the cathode voltages.
If you put your (-) probe on the cathode you will get a better result. Then calculate the difference from your measured cathode voltage.

[The Dude]

I'd start by pulling tubes working your way one direction or the other in the signal path and see if you can determine which stage the hum is coming from.

[Helmholtz]

60Hz or 120Hz?

[Randall]

OK, so it seems the PI grid voltages were a red herring.

The audible tone is 120Hz, but it looks like this.

Only power tubes, PI and Reverb recovery tubes in the amp. Reverb pot all the way down, and I can hear the hum. Grounding grid pin 7 on reverb tube silences the hum, but the wave is still there. Pulling the reverb tube silences the hum, but the distortion is still there. Pulling PI tube shows the distortion, but to a lesser degree. Changing the PI tube has no effect.

Is that wave an oscillation? I have tried neatening up the lead dress, but I don't see any change. This is strange.

[Randall]

We are talking about a 1965 BF Vibrolux Reverb.

[Chuck H]

May seem counter intuitive, but...

This could be a problem with DE-coupling the circuits. I know you replaced the filter caps, but are you certain they're doing their job? I've installed brand new filter caps straight from Mouser and they were stale and bad. Ok, it only happened once, but it happened.

[Randall]

I did measure each F&T cap before installing, but I know one could still be bad. Not sure I actually understand DE-coupling idea.. What is there to do about this possibility?

[g1]

What kind of voltage level is that scope shot of the HF oscillation? Sometimes if you crank the scope gain enough you will see that kind of crap on the ground.

[Randall]

The burst is about 20mV p-p. And remember, it's audible. When the burst goes away, the noise goes away.

[Helmholtz]

The audible tone is 120Hz, but it looks like this.

Only power tubes, PI and Reverb recovery tubes in the amp. Reverb pot all the way down, and I can hear the hum. Grounding grid pin 7 on reverb tube silences the hum, but the wave is still there. Pulling the reverb tube silences the hum, but the distortion is still there. Pulling PI tube shows the distortion, but to a lesser degree. Changing the PI tube has no effect.

Is that wave an oscillation? I have tried neatening up the lead dress, but I don't see any change. This is strange.

What are scope settings?

What do you mean with "wave"?

What "distortion"? Difference to "wave"?

Scope pic looks like bursts of oscillation.

Decoupling is the opposite of (unwanted) coupling.
Power supply filter caps serve to provide decoupling between amp stages by shorting signal riding on the supply to ground.
If ESR is too high or cap connections are bad, signal from a following stage may couple via the supply into an earlier amp stage and cause instability.

[Randall]

Scope settings are 5 mV/div and 2 mSec/div.

Sorry for mixing terminology. By wave and distortion I meant the burst of oscillation.

So what can/should I do about this?

[Helmholtz]

When the burst goes away, the noise goes away.

When does it go away?

[Helmholtz]

I did measure each F&T cap before installing, but I know one could still be bad. Not sure I actually understand DE-coupling idea.. What is there to do about this possibility?

Tack a good cap in parallel with existing caps. Use a good grounding point for the negative end.

[g1]

Can you clarify the following two statements, they seem contradictory.

Grounding grid pin 7 on reverb tube silences the hum, but the wave is still there.

The burst is about 20mV p-p. And remember, it's audible. When the burst goes away, the noise goes away.