Thanks to Robin’s help, I think the noise has been located and eliminated. It seems that it was a poor connection in the socket of pin 8 on the power tube. Tightening that socket “clip” seems to have taken care of the strange static. Apparently my previous efforts to tighten up those sockets were not sufficient.
Thanks to all who chimed in with suggestions and advice – I’ve learned a lot more about how the amp works as the result of chasing this noise around!