[MrFixr55]

Hi Arran,
I had posted 2 diagrams. The first one with the red boxes is the Philco 650. Fortunately, it was the output transformer that failed. Hamilton measured his interstage transformer and it was good.

Ron Ramirez has written a very good article on correcting for a failed interstage transformer primary. I don't know if it works if the primary of the interstage transformer failed in this set. A 42 is used as a driver because the grids are driven positive at peaks in AB2, therefore not just voltage but power must be generated by the driver stage. I suppose that RC coupling could be used but there may need to be a large cap to do so.

The second diagram is of a Heathkit W-6M Amp. This is a very powerful amp using Class AB, fixed bias from a separate supply as opposed to being derived from dropping resistors in the B supply return. They did not use an interstage transformer but used a 12BY7 in cathode follower mode. (I think that 12BY7s were popular as vertical oscillator / output tubes in mid 1950s TVs, therefore likely could put out enough power to drive a Class AB Amp. Heck, this beast puts out 70W RMS, 140W peak.) I am posting the W-6M Manual for anyone interested. The specs and what heath did in the manual to prove these specs are amazing, especially for 1957.