I finally finished one of my 6L6 monoblocks. Aluminum was tough to polish, but at last I was able to wire the things. The other one is still sitting on the bench, not quite done.
First, some photos. Then, the bad news :)
Here's my problem. The amp started up fine, fed by a discman into a decent speaker. Upon playing the music, though, this horrible distorted bass rumble started blaring. I shut down the amp and it "killed" the home-theater voltage regulator thing it was plugged into, resetting the TV, cable box, etc. Surprisingly, the fuse didn't blow.
The amp, ignoring the noise (which I suspect is motorboating), sounded pretty good otherwise. The cymbal hits on Miles Davis' Nefertiti album came through pretty decently.
Back at my bench, I put on some gloves, tipped the amp onto its side, and measured some voltages. Or tried to. Voltage on the first motor run cap, the 100uF main B+ reservoir, started at 450V and rapidly rose to above my crappy tester's 500V limit. I then stupidly blew up said DVM when I put the leads from one side of B+ AC to the otherwords, probably around 1000V. Damn.
I'd had the bias pots at around half way. Without being able to measure tube current, I turned the 20-turn 25K trimpots a few times in one direction, then a few times in another, without getting much difference in sound.
One last thing. Turning the amp on and off back in my workshop didn't blow any fuses. I left the CD player off, but still plugged in, and turned on the amp. It was quiet, just the faintest bit of hiss and B+ ripple noise. I tapped the side of the chassis firmly (as it was vertical). The rumbling hum noise started up again, building pretty quickly to a steady loudness. What the hell? I made sure that not too much stuff was dangling around, even super-gluing a stiff piece of wire to the top capacitor to keep it from vibrating.
My questions - if it is motorboating, is there anything I can do? I made sure not to mess up the wiring of the OPT, but those long runs of DC between those last 22.5uF motor run caps and the input stages might be a problem. My main issue though is that I've never had this problem of crazy noise, and don't know what could be causing it. Someone suggested that tapping the bias voltage off the B+ could lead to a feedback loop, could this be it?