'74 SFDR reverb issues/poltergeist...

[55hz]

Sometimes the reverb works great (albeit very "surfy").

Sometimes, out of nowhere, it screeches at an unbearable volume. If I turn the "Reverb" to "0" it stops. This is intermittent.

Today I turned my amp on, it was humming loudly. Even with no inputs plugged in or either volumes turned past "0". I finally turned down the "Reverb" knob and the humming decreased, and then went quiet, when I got to "0" on the "Reverb" knob.

Any hints/ideas/clues as to what is going on here?

 

[guiltless]

My first thing that I check is the reverb driver tube. It's the easiest thing to check as you don't have to remove the chassis. Also, try cleaning the tube pins and the socket itself.

If that doesn't work, a chassis pull may be necessary.

 

[Fritz_D_Cat]

I just serviced a DRRI with exact same issue. Leaky (one way) .0033 coupling cap in reverb circuit set a bad cap in doghouse to oscillating. Check the filter caps.

 

[55hz]

Thanks, I'll try that.