Just a short follow up to an old discussion. The static noise that Robin helped me eliminate eventually returned, and I decided to replace the power and rectifier tube sockets. The socket for the pre-amp tube fits quite snugly, but the power and rectifier tubes seemed somewhat loose in the original sockets.

I replaced the sockets with Belton “fork type” sockets (instead of the sleeve type sockets that came with the kit). The tubes fit much more snugly in these sockets.

Since replacing the sockets, the noise has not returned at all, even though I have been playing the amp almost daily for the last couple of weeks. I wanted to share this info in case anyone else runs across a similar problem in the future.

Again, I would like to thank everyone who offered help in tracking down the problem!