THe "250 Volt" rating for the 6B4G is the voltage difference between the plate and the filament / cathode. (V P-K) With the 305V on the plate, and the "cathode" sitting at around 50 to 55V, the ratings aren't being excessively "stretched". There is also a few volt's drop from the two "center tap" resistors, s there is a tad more safety margin.
The "classic" RCA tube manual design runs the 2A3 (and 6B4G) at 250V P-K with 60 mA plate current, just "nailing the" 15 watt PD rating. I'm sure, that in real-life situations, this often got exceeded by 5-10%, with no ill efects.
Newer imported
СДЕЛАНО В СССР . Sovtek, E-H, J/J, etc single-plate 6B4Gs, 2A3s, etc, can take higher voltage and current than the vintage/classic/NOS tubes. Treat the old ones more gently, as they are expensive and getting harder to replace, the Sovteks, J/Js, etc are reasonably priced and readily available. The 1950s vintage Russian Svetlana dual plate 6C4C (6S4S) and 2C4C (2S4S) are more delicate.
These have the classic twin plate construction.
Zey vork wery vell in Tupelov TU-4 samolyet, tovarisch, sank you for the ze werry nice B-29s.
(
how's that for bastardized Russlish...)
/ed B