[RichG]

The ZTO is a 6T40Z chassis model 600. I think 1955 vintage. And yes, I was lucky to get it at a good price along with 3 big boxes of tubes that the seller just wanted gone. It was a good thing because I found a couple of replacement tubes for the ZTO. The Tolex was in pretty good shape, just a good cleaning and a spruce up with Meltonian creme polish. The shoe polish has a pleasant odor that doesn't linger. Mine uses the unobtainium 1L6 tube and it was dead. I did find a guy that replaces that tube with a solid state tube of his design. Somewhat similar to the old Fetrons that Heathkit used to sell as replacements for their VTVMs. I think my 1L6 replacement used to be a 12AU7. He cuts the tube off at the base, cleans out all the guts and replaces them with small surface mount PC board and then glues the tube back together. The solid state tube works great.
I know what you mean by the very different AVC lines. I scoured some books on various AVC schemes but did not run across this particular method. Yes, I did know that the center tap is not B-. I've run across similar negative biasing techniques like this one.

Rich