Fender Pro Reverb AA165

[goldtop5]

I have a vintage Fender Pro Reverb (model: AA165) that's making a loud squealing noise. It goes away when I remove the last pre amp tube (12at7), but get no sound. Any hints?

Thanks!

[Jazz P Bass]

Did the problem suddenly arise?
If anyone was goofing with the amp, I would look at the output transformer primary wiring.
The blue & brown may have been reversed resulting in positive feedback.

[Enzo]

Agree^^^

Has the output transformer been replaced or rewired?

[goldtop5]

Agree^^^

Has the output transformer been replaced or rewired?

I replaced the new transformer with the original one. However, I did add extension wires so that they would reach both tube sockets and center tap connection on the circuit board.

[goldtop5]

Did the problem suddenly arise?
If anyone was goofing with the amp, I would look at the output transformer primary wiring.
The blue & brown may have been reversed resulting in positive feedback.

No, there was a newer one, but replaced it with the original one upon request of the owner. I will try reversing the blue and brown wires to see if that helps.

[Jazz P Bass]

If you are curious how to tell, set up the amp on a dummy load with a signal injected in the front end.
Measure & monitor the amplitude of the output sine wave.
Now power down, drain the caps & disconnect the feedback resistor.
Power up at the same signal input level & measure the output.
If the amplitude goes up, then the OPT is wired correctly as you had negative feedback working.
If the amplitude goes down, then the OPT is wired incorrectly, as you had positive feedback working.

Or simply switch the wires & see if the squealing stops.

[Enzo]

Or lift the NFB and see if it stops.

[goldtop5]

No, there was a newer one, but replaced it with the original one upon request of the owner. I will try reversing the blue and brown wires to see if that helps.

What I did, I followed how the previous output transformer was wired. I reversed the blue and brown wires like you said and the amp came back to life. Thanks!