The unit uses a 6X4 dual diode for the power supply rectifier. As I was doing the final cal and buttoning things up, something started to go snap snap zzzt. After tracing things down and measuring everything, I found that the pins for the 6X4's plate and cathode were shorting together in the tube socket itself. The socket is one of those cheap dual-layer wafer sockets, and it apparently has some carbon buildup inside the wafers. It has a minimum value of 60 ohms from plate to cathode (tube removed.) Just goes to show you that anything can fail on an old device. I've never run across this problem before, but now it's something I'm going to be checking for. Once I get replacement parts, I'll drill the old socket out and look at the wafers to see if I can spot the problem.