Time for a little humility!
I've been doing more calculations. Reading, calculating, reading, calculating...
I now see what the problem with my amp design is. It's the B+ voltage required because of the tube I chose. I'm looking at potentially putting up to 4000 volts peak to peak across the primary of a 20K custom tranny to get 100 watts! WOW! The even bigger problem is the increased B+ required due to the plate characteristic curves of the GM70. The tubes have two problems: high plate impedance requiring a high impedance transformer requiring a high AC voltage for rated power; and, the zero grid voltage plate curve intersects any reasonable load line at several hundred volts (up to 500 volts) which increases the B+ that's needed.
The 10K Edcor model CXPP100-MS-10K would need only 2800 volts peak to peak for 100 watts assuming a sine wave input. My design would allow 2300 volts swing and used with that tranny would yield about 66 watts, but the tubes would see a low impedance load they wouldn't like and linearity would suffer.
However, I can't even do that.
The the zero grid bias plate curve is at 450 volts where the load line intersects, which forces B+ way up beyond what other tubes would require. There's a 450 volt "window" where the tube is non-conducting, pushing up the required B+ to unacceptable voltages. I need a transformer that can handle a very high voltage. I'm learning a lot!
When I talk to Edcor I now know a lot more about what I need and what the problems are. Now I see the reason for no secondary taps and no UL taps. Bringing out those taps probably reduces the breakdown voltage.
I'm "getting" this now I think. It will be interesing to see what Edcor can do. I'm thinking the lower the impedance the primary is the less voltage I'll need to put across it and I can drop the B+ a little. Maybe a 15K tranny with no secondary or UL taps could be made to withstand a lower B+ and still extract a resonable amount of power from the tubes. I'm running some calcs now to see what B+ would be needed for 50 watts at 15K using a GM70.
If Edcor can't get me the B+ required for 50 watts at 15K I may change my tube selection. $170 mistake counting the sockets. I could unload them for $100 on ebay and lose $70. Fortunately the power tranny has a half voltage tap and can be bucked by a small transformer to make another amp with different tubes. Maybe like a parallel pair stereo KT120 oddwatt?
Save me Edcor!!!

If they can't it still appears I can go single ended and get 28 watts with a Hammond 1638SEA. It's specifically made for triodes, has no UL taps, and is hi potted to 2000 VRMS (over 5600 volts peak to peak).
Does anybody see a problem with that (other than the public humiliation of changing my design!)?
