Half volume until standby flipped - Acoustic Control G60T

[MTill]

Hi all,

I'm hoping for some help with my Acoustic Control Corp G60T tube amp. When I first turn it on, it works at normal volume. Then after a minute or so, it will go to about half volume with some loss of bass although that could just be because the volume is lower. If I flip the standby switch briefly to standby and back, it's back to full volume. If I play for a while, 10 to 15 minutes, flipping to standby when necessary, it starts staying in the normal volume mode for longer and eventually, sometimes, it seems to stay in the normal volume mode. Any ideas? I've replaced all the tubes without any real change. What should I do next? I've a mechanical engineering background with a fair bit of electronics exposure but not much practical experience.

Thanks,
Mark

[g1]

Put a patch cord from preamp-out to power amp-in jacks and see if it still has the volume drop outs.

[glebert]

Maybe the standby switch is going, either corroded contacts or weak springs. You could temporarily bypass it (please study up on electrical safety before attempting).

[The Dude]

Here's a link to the service manual.

https://www.manualslib.com/manual/10...stic-G60t.html

[MTill]

Thanks very much for the replies.

I tried jumping from preamp out to power amp in an it still behaves the same. I may try bypassing the standby switch but it doesn't seem likely to be the issue as it always behaves the same and it doesn't make a difference if I switch back from standby quickly or slowly.

I do have schematics. Let me know if they would be helpful to anyone.

As background, I bought the amp in the early 80's as a head and built it into a combo with a JBL K-120. Dovetail jointed Oak inspired by the Mesa Boogie. Sounds wonderful when it's working right!

The delay in responding to your replies was because I thought I would get an email when there was a reply.

[Helmholtz]

I'd start with measuring B+ and both power tube screen voltages in failure mode.

[Helmholtz]

When you can measure a voltage across a closed (standby) switch it's bad (same with a fuse).