I must have been waaaaayyyy too sleepy, I saw 6BQ6 but read it as 6BQ5
Duuuhhhhhh.......
So, I'll erase the past, and start over again.
The 1965 ARRL radio amateur's handbook had a 6BQ6 modulator construction , it ran a B+ around 300-325V, and the OPT was 4K CT. Standard pentode mode, cathode biased. Good for around 40 watts.
If you can locate some EL36s, so much the better...
FWIW, the 6BQ6 and 6AV5GA have nearly identical operating characteristics, though the 6AV5GA apparently can take higher power operation in stride.
For a PPP output stage, a 1.9 or 2K CT OPT would be good, and give you about 70-80 W out.
Going along these lines, probably just about any decent horizontal sweep tube will do the job. I've got several 6GT5s and 6GE5s, for future experiments in sonic terror
Any decent driver / LTPI should be able to handle either 6BQ6 or 6AV5GA .
/ed B