Fender Pro Reverb - Reverb Squeal Issue

[misterc57]

I am getting a piercing high pitch squeal when turning the reverb pot above 1. Both with and without the reverb tank connected.

My first thought was the pot but I wonder if there may be something else to look into.

Thank you!

Fender-Pro-Reverb-AA1069-Schematic.pdf

[Chuck H]

I suspect a microphonic tube first. After that I would suspect a ground fault in the recovery circuit.

[Helmholtz]

Or a bad filter cap at node D.

What happens if you short the grid of the recovery tube to ground?

[Chuck H]

Ok. Changing my reply to microphonic tube, filter cap at D node and then ground fault. I forgot that these Fenders share like phase stages on the same node. Thank you Helmholtz.

[pdf64]

Another possibility is the V4 cathode bypass cap being bad.

[misterc57]

Another possibility is the V4 cathode bypass cap being bad.

That was it. Horrible squeal is gone.

That cap was bad. I am going to check all the other cathode caps and replace them.

More work is needed because with reverb ON, the note sound quality is not right, kind of fizzy sounding. Reverb OFF notes sound correct.

Thank you, MarkO

[pdf64]

V4 is about the only cathode bypass cap that could cause a big issue when it fails. Due to it being shared by 2 gain stages wired in cascade, a positive feedback loop somehow then forms. It’s shoddy design really, but it works fine for decades, and saves a few pennies and a bit of board space.

[misterc57]

Swapped V3 and the fizzy sound is resolved.

Thank you everyone!

MarkO