I think I mentioned this to you before, but I am in the market for a pair of beefy output tramsformers.
Output valve STC 4212 and friends.
B+ 1500v
Max current 160ma.
Maybe looking at 80W at 25Hz as a target.
I think 7k looks like a good starting point, and there will be 6A DC available via the heater supply if you feel that a offset winding is of use.
The B+ supply is done and proven with 833's, likewise the driver stage is just about sorted, again having proved it can push a 833 into +50v on the grid.
All that is missing from the equation is the bit between the output valve anode and the loudspeakers
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