SF Twin Squealing When Switching Pickups

Hi All. I'm hoping I can get some guidance and reassurance from you amp gurus. I got what I believe to be a 1975 Twin Reverb yesterday off of CL. Couldn't detect any problems when I tried it out or played it at very low volumes last night. This morning I had a chance to open it up a bit (nothing crazy, just rehearsal volume) and ran into some trouble.

Using either of the Vibrato channels. with the Master Volume on 10 and the pre-amp volume between 2 and 3, I get a loud crunching sound when I switch pickups using the selector switch on the guitar, followed by a feedback-like squeal.

It's not feedback from the guitar - it persists regardless of the guitar volume being turned down to zero. It only stops when the MV is turned to zero. It also doesn't happen with other amps - pickup switching is silent - so I'm assuming it's isolated to the Twin. (I haven't tested the regular channel yet - I've put my dog through too much already.)

Edit: I tried it on Channel one and had the same problem. Also, digging in hard on a string causes the same pop/crunch sound followed by the squeal.

Any ideas? How bad is the news? Will I ever be able to trust this amp for gigging? Please tell me it's not a transformer. Thanks in advance.

- Coop

Try changing the V4 preamp tube, if your problem is with that channel only.

Thanks vibrosonic. Turns out it's in both channels. I just added an edit above.

Well, here's what I usually do with a new amp in the fold.
Usually pull all the tubes and give the sockets a shot of contact cleaner and re-install just in case there's any crud build up.
After that, usually just mess with it to find any sonic oddities and such.
Sometimes I've found that power tubes about to go out will squeal, but sometimes microphonic pre-amp tubes will with increased input gain.
Maybe Wally, Celeste, or Muchxs will see this and have a better solution for your problem.
Sorry I couldn't be of more help. Been lookin at SF Twins lately myself, so will follow with interest.

Thanks bunny. One one hand, I'm hesitant to start randomly cleaning and reseating stuff because I'd like to know what exactly is causing it (in case it ever starts doing it again during a gig), but I just might have to go that route. The seller said it has been recently retubed, but I think I'll start with the power tubes and then maybe the reverb (although I'm not sure if the reverb would cause it in both channels.)

In case anyone is interested, I think I isolated it to a bad power tube, just like bunny 7 suggested. The seller had recently retubed these with GTs, and they do look pretty new, but one is really noisy. I replaced them all with a borrowed a set of four and couldn't replicate the trouble.

Anyone know if these need to be replaced in matched quads or will matched pairs do?

Hey Coop47, glad you found a solution. I'm sure someone else might have a more technical explanation, but using a set of matched quads would be the most straightforward answer as they would be a matched set to start with.
Using matched pairs could work providing both sets are in the same range with each other, but that might not be as easy to find at a retail store.
The safe answer would be quads, imho.
As usual, bias should be set accordingly to the installed set of tubes.
Glad you got it working.

try taking all the pre-amp tube covers off, then while running tap each pre-amp tube with a wooden chop stick or a new2 long pencil, when you hit a bad tube you will know it.

Was the amp ever re-capped?

Thanks bunny7. That's consistent what what I've been able to find online, and I have a set of JJ's in mind for it, as well as a biasing. It sounds slightly gritty to me for a twin; I have a feeling it might be biased really hot.

Inventour, the tapping is how I found it. I don't know about the cap situation, but will check that out eventually as well. Thanks.