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my build is working really great for what I do. I mostly use it at moderate volume on the first channel. today for fun, since nobody was home, I just spent about an hour with it totally cranked - great sounds. I notice that on the TMB channel I had a very high pitched "harmonic" type sound that was almost squeal. It wasn't feedback it was tied to the notes I was playing up the neck on the guitar. This was happening when I had the gain way up around 8 and the volume on 10. When I backed off the gain it went away.

Has anyone heard this and what is it? Should I have switched off the boost circuit? Is it part of the amp with the gain cranked up like that? I honestly dont think I would ever use it with that extreme amount of gain anyhow so I'm not too concerned but just wondering what it is.

Thanks

My sIII (my first build and therefore a little sloppy) also had a squeal when the gain was up and boost on... until I moved some wiring around, and shortened some of the runs.

You can try using a chopstick while the amp is powered up to move wiring a little and sometimes just a small change in positiion makes a big difference. My Triwatt also had the same issue, which iwas also solved by slioghtly moving some wires and using zip ties to hold them where the squeal was gone.

AJC

It could be a microphonic tube feeding back from the speaker vibrations. Mine did that. It went away when I moved the head off of the speaker cab. I swapped out V1 and cured it.

LeeMo

For the suggestions.

Did you try swapping the blue/brown leads on the output tubes? That sometimes helps.

Also, did you ground the output jacks to the chassis? That can help in some cases also.