[bandersen]

Replacing the 6AT6 seems to have helped. However, when I put the original back in, it seems better now too. I did clean the tube sockets with DeOxit right at the beginning but maybe it wasn't thorough enough?

Or maybe an ugly possibility that there is some contamination in the tube socket. I'll eliminated every other possibility before I even think about replacing it.

Another observation (clue?) - the hum/noise slowly changes over time. Perhaps in sync with the slowly traveling wave in the horizontal. I assume that's some 60Hz signal interacting with the 29.97Hz vertical oscillator.