(01-14-2018, 06:08 PM)slider313 Wrote: Glad you fixed it! The first thing I do, to a vintage amp, is replace the electrolytics; if they're original to the amp. They can cause; a voltage drop, popping, hissing, motor boating, etc. I was restoring a Gemini I, a few months back, and the first section of the cap can was 100v lower than the schematic. I disconnected that section and clipped in a temporary 40uf@500v and the voltage came up to spec.
In this case, all the components I replaced I had just replaced a few weeks ago. All tested as they should, and all voltages were correct. But I thought that I had isolated the issue to that part of the circuit, but it turns out this issue is more devious than I had originally thought.
Well, as is so often the case, I spoke too soon. The amp sounded great for about 10 minutes this morning, and then the exact same noise was back. On both channels. It seems like the issue materialized either after time (it was 10-15 minutes) or volume (I was continually going just a little bit louder about every minute).
It seems to be worse with the bridge pickup over the neck pickup. That could be the result of output or voicing. Right now, I am thinking parasitic oscillation. I've mentioned a few times that the first tube does not have grid-stoppers. So that is definitely something I will add the next time I crack the amp open.
But I'm shooting in the dark right now, so I guess I'll just think about it and see if I can't come up with some way to confirm a diagnosis.