Yeah, this is part of the adventure when you stray off the beaten path.
Yup. I've been waiting for something like this to happen. I've gone over the wiring half a dozen times and can't see any problems, but reflowed a lot of joints just in case. At this point any more disassembly-then-reassembly is probably only risking more harm. You can only fiddle with wires so much before they break or loosen.
All voltages check out, and the amp sounds fantastic - can't hear the hum except in silent passages. I measured the DC filament "pos out" voltages a few moments ago. Right (offending) side: 9.84 VDC out (0.355 VAC). Left (ok) side: 9.9 VDC (0.345 VAC). And that's at 116.5 VAC mains - they'll be a bit higher @ 120 VAC by way of the variac I've been using to bump up my mains. And I'm within 4% of Jac's filament votage target for the EML 300Bs, so that checks out too. On the off chance: could the 1/3 V of AC on the filaments cause noticeable hum?
I have one Hail Mary left: the hum sounds a lot like the motor hum of one of my aquarium filters that I recently got going again. I've read that electric motors can put junk on the lines. But I'm not holding my breath: I'm powering the amps through a DIY common/differential mode mains filter that is supposed to filter out most of that kind of noise (and it's not the mains filter itself - I tried swapping it out).
If its not the aquarium thingy, I suspect there's some hidden shard of aluminum somewhere that's making a connection that shouldn't be made. Drilling holes in the chassis for the chokes and a couple of new terminal strips produced a lot of aluminum shavings - and some of them went flying off the drill bit. I did a pretty thorough job of shaking them loose and hunting for them, but clearly not thorough enough: I found 5+ tiny aluminum shavings this evening. Three had found there way into the bearing grease of the BeeQuiet attenuators! Which I thought was a most improbable place for them to end up. If they can find their way in there, they could be anywhere. And they are almost impossible to spot b/c they look identical to the chassis. So I think the best way forward (if it's not the aquarium filter) is to rebuild it. Maybe even with new filament reg boards if those can be ordered - I did lot of playing around with them trying to get the amp dialed in for the EML 300Bs, and I don't think the traces can take any more rework and are probably on the verge of failing already.
On the upside: I can make the chassis bigger to fit all the stuff I've added.
At the very least that will make debugging a bit easier. As it stands, I have to remove the giant 22 mF caps every time I want to investigate something.
cheers, Derek
