I’ve obviously been taking my time with this project, but I never intended to build half of it twice!
I was getting occasional, unexplained changes in B+ voltage readings since the first power-up, but the preamp behaved pretty consistently, so I let it slide. When I started looking at the power amp it was obvious that the signal wasn’t balanced, and I eventually determined that something fried one of my 6BM8 pentodes. After a few fried tubes and much inspection & general poking around, I realized that there must be an intermittent connection to the tube and that the connection was in one of the sockets.
Back in the day, when I was buying tubes & parts that might be useful some day, I found a guy in Lithuania on eBay who had acquired lots of Soviet parts and equipment when it fell off trucks as the Russians left for home (or something like that). I got lots of good deals on parts & tubes from him, but these ceramic sockets are a bust. They might work OK most of the time, but can’t be re-tensioned when the fit gets loose. So, after just a little wiggling (like inserting & removing a tube a few times) the socket holes can — and do! — get loose & intermittent.
You can see here that once the contacts on the left get loose, you have no way to get somethng in there to re-tension (smallify?) them.
So now I’ve replaced all the ceramic sockets & re-wired the heaters, but I need at least a sketch for a new layout of the rest before I finish rebuilding. Fortunately, the replacement sockets fit in the existing chassis holes, but the the orientation of the pins is rotated 90 degress relative to the screw holes so all the point-to-point work around the sockets has to be re-done from scratch.