AA165 Pro Reverb with buzz in reverb

[Randall]

This amp comes in with a complaint of an annoying buzz in the reverb. Noticeable and a bit annoying on 4 on the Reverb control, and downright unacceptable on 10. Swapped tubes and reverb tank and cables, checked all grounds, resoldered both drive and recovery sockets and associated wiring. Noise stops when grounding recovery grid, and when putting reverb control full off.

I have 302v on the recovery plate and 2.1v on the grid. The grid resistor measures close enough, as do both cathode resistors, The cathode caps have recently been replaced.

This should be relatively simple, what am I missing?

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Tone Meister]

Did you service the RCA connectors on the rear panel for the footswitch? If not, try loosening each connector and check underneath for rust or oxidation. Wire brush if necessary. Then snug those connectors so they grab another bite into the chassis, which will improve the grounding at that point.

[Randall]

I unsnugged and snugged those connectors down tight and hit them with DeOxit, but this is a very clean and shiny chassis, and the grounds all test 0.2 ohms. No difference.

[The Dude]

I know you tried a new tank, but is it the right tank? I'm just wondering if someone installed the wrong tank and you replaced it with the same part number tank.

[Tone Meister]

I know you tried a new tank, but is it the right tank? I'm just wondering if someone installed the wrong tank and you replaced it with the same part number tank.

It should be 4AB3C1B

[Randall]

"It should be 4AB3C1B"

I have not pulled the one that's in it, but the one I am bench subbing is a 4AB3C1B from a Twin Reverb.

[pdf64]

How about the footswitch? eg is one connected, if so try unplugging it.
If no improvement, how about the ecaps? Try tacking another across the V4 cathode resistor.

[Randall]

There is no footswitch involved, but grounding the footswitch jack silences the noise. The ecaps are all recently replaced spragues. Jumpering V4 cathode makes no difference.

[g1]

Not sure what you mean about jumpering V4 cathode?
Have you tried grounding V3 grid?

[Randall]

"Not sure what you mean about jumpering V4 cathode?
Have you tried grounding V3 grid?"

Jumpering a fresh cathode cap accross the existing one as suggested above. The buzz is there without a tube in V3.

[Randall]

Well, OK, I did find a bad 22/500vfilter cap. They had been changed out before, but the 22uFs are Illinois Caps, the 80 uFs are big and blue installed face down, so I will assume those are Spragues. Changing that bad one out helped, but it still is buzzier than it should be. Maybe all filters need to be changed to my standard F&T? But I only see between 1 - 9 mV fluctuating ripple on the recovery plate. I've done all I know how to do here.

edit: when I connected to the original pan, it quieted down. Now the background noise from the reverb control is less than from the volume control. So, I am going to suggest at least replacing the 22 uFs.