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I completed my Trinity OSD kit. When I play it through an Avatar 112 cabinet loaded with a Celestion Vintage 30, I get a weird oscillation when I play the 6th string, 5th fret (A). It's a bass tone that's louder than any other note. It doesn't happen anywhere else on the neck, only the 5th fret, 6th string.

I plugged it into my Marshall 112 cabinet loaded with a Celestion 70/80 and it doesn't happen. Then I hooked up my Trainwreck style amp to the Avatar cabinet - same guitar, same cabinet location and it doesn't happen. It's only the combo of the OSD playing through the Avatar cabinet that makes this sound. If I play loud with a strong pick attack the oscillation doesn't stop - I can mute the strings and it continues. I have no idea of what's going on here. Any ideas?

So, I played for another hour tonight with the OSD and Marshall cab - no issues, in fact thr OSD is one of the least noisy amps I've ever played. Then I played the Trainwreck into the Avatar cab and noticed an increase in hum and a couple of crackles. I suspect something is up with either the Avatar wiring or the speaker. I'll have to open it up for a look-see.

I found the problem. I had positive feedback instead of negative feedback. I swapped the output wires on the plates of V4 and V5, problem solved. I had always assumed having them the wrong way around would result in a squealing, noisy amp that constantly oscillated. Not so in this case, it only happened at a certain frequency. I guess I've always been lucky with my other builds and never had to swap the leads before.

Thanks for posting this Koop. I'm building an OSD now and this is really good to know.