Acoustic G60t squeal

[dmartn149]

My Acoustic Control G60t has started to make funny sound. When I turn the gain up on the distortion channel it sounds kind of like a theramin or like letting the air out of a balloon while stretching the mouth tight. It goes away while I play, but comes back as soon as the note fades away. I tried changing tubes with no improvement. I did change some caps in that channel a while back to alter the freq response, but it was working fine till last week.
Any Ideas?
THANKS!

[jrfrond]

Does it happen WITH the guitar plugged in and guitar volume turned up? If so, turn the guitar volume down and see what happens.

Sounds like pickup squeal to me.

[Euthymia]

I've seen this in every G60t I've had in the shop, and what it probably is is the power supply filter cap for that stage.

The one in the can with a gold cardboard cover.

First one I worked on, took me a long time to find it, but it drove home the fact that a bad power supply cap can cause problems other than just ripple/hum.

I knew about motorboating, but wasn't so versed in high-frequency squeals as a symptom of poor PS decoupling.

[Zouto]

It's rather common with those amps, the coaxial gain/volume pot is one of the culprits (cross talk), also the related filter cap and coupling caps.It's also important to check cables and ground connections.Last not least a proven good 12AX7 is important, try a RI Tung-Sol or a Sovtek 7025WB, stay clear of whistling JJs.

[dmartn149]

It doesn't matter if a guitar is plugged in or not, I've tried different tubes.
Is that cap with the gold cardboard, the one sitting next to the choke? It looks like two caps in there.
I'm also having trouble with biasing the new power tubes. First time I tried I got to about 28ma on the OT shunt, then it just started running away. I cleaned the pot and tried again with the same results. The squeal started with the old tubes, and is present still even with the new tubes biased as low as it will go.

[dmartn149]

I've seen this in every G60t I've had in the shop, and what it probably is is the power supply filter cap for that stage.

The one in the can with a gold cardboard cover.

Euthymia, You hit the nail on the head. I used jumpers to temporarily hook up another cap in parallel with one of the caps in the can, and the squeal is gone.
Thanks!
PS I think maybe the runaway bias may have been while it was squealing. Biasing fine now.