You need two seperate cathodes. Two Bendix 6754 would do it. And give ~45 second warming time, but they are not a mainsteream valve. If you go directly heated, we'd need a filament winding at each end of the plate winding. Cruise Triode Electronic's Altec page and look at the full bridge on the 1570. It is made from 5R4's.
All valve is nice, but in this case additional power Tx expense. We could do a 250-0-250 plate winding...all on a sudden, instead of a $28 power TX, it is nearly double that.
regards,
Douglas