Gibson 20-T squeal

[fester gomez]

howdy, I have a late 50's Gibson 20T amp that squeals when the tone control is turned up, it isn't feedback from the pick-up, I believe it is an parasidic oscillation. Anyone have any experience with this amp.....thanks for any help!

[pdf64]

Maybe a tube has become microphonic.
If it's this model http://www.webphix.com/schematic%20h...amps/ga20t.pdf then the 5879 is most likely so try a 'known good' spare.
A parasitic oscillation only occurs when another signal is present, eg when your guitar is making a sound through the amp.
What you've got is regular oscillation.
Pete

[Chuck H]

+1 on the microphonic tube. That's the most likely cause. The second most likely cause would be bad filter caps. If the caps aren't "decoupling" well there can be positive feedback bleeding into the signal chain from the power supply rail. Third most likely is a bad ground. Age and general atrophy may have degraded one or more connections. The fourth most likely cause is that someone worked on the amp and NOW it squeals. Meaning something as yet undefined has changed and the amp doesn't like it. Fifth would be component failure. leaky caps and drifted resistors are common in those amps.

[TomCarlos]

Gents.... I have another thread open for the rebuild of a Gibson GA 20T. But since I saw the keyword "squeal" I thought I would chime in here. I'll add findings to my other thread.

One of the two problems I have with the amp is the squeal. I have removed the tubes from Channel 2 (5879 and the 6SQ7).

As you can see from the photo, the Output Transformer (thinking it is the original) is mounted to the bottom side of the chassis, in the same direction as the Power Transformer. Intermittently, there is the loud squeal. It first kicked in when I had the guitar at 8 (humbucker), the Tone at 2 (towards the Treble side), and the Volume at 7.

The squeal goes away when I put my hand across the strings OR if I touch the chassis. I am also thinking the Output Transformer needs to be moved away from the "Input" end of the chassis?

Any hints?

Thanks, Tom

[guitician]

It looks like someone moved it from the power transformer to remove hum? Spliced on some extra wire too. I would think moving it back to it's original position would help.

[TomCarlos]

I removed the Output Transformer from the chassis... no difference.

I took a look at some Fender 5E3 amps. The OT is right next to the PT. So if we mount the OT back on the chassis, it makes sense to keep it away from the input tubes and to shorten the leads feeding the OT.

The problem traces all the way back to the input. I can see that on the scope. I am going to see if shortening the leads from the jack to pc board to tube makes a difference. I'll post results.

[g1]

Try a ground jumper from your 1/4 plug shell direct to chassis. Just to eliminate any odd grounding issues.

[TomCarlos]

Yes Greg.... I tried that. I even connected a floating 1/4 plug and moved the ground to a couple spots. Same results.

Is it possible that an amp can have some type of parasitic oscillation that feeds all the way back to a first stage tube? I am wondering if I am looking in the right place???

The search continues.

[guitician]

Is it possible that an amp can have some type of parasitic oscillation that feeds all the way back to a first stage tube?

Yes, if there are any currents running through the grounds small resistance and acting as a feedback pole. That is why a ground buss is good way to handle grounding, it keeps the resistance low.

[TomCarlos]

Gents... sorry to do this... it might be too confusing with two threads. I am going to move this back to Gibson Ranger GA20-T Rebuild.

Using my scope, I think I have excessive ripple. Will write more there.

Gibson Ranger GA20-T Rebuild

Sorry!!