Other than 6B4G, there are 6A5, 6A3, and 2A3 (requires 2.5 V @ 2.5A filament per tube. 2A3 and 6A3 have the old stule 4 pin "UX4" base.
Going a bit further, the 300B (doesn't really have to be the pricey and rare WE300B) is a nice power triode, capable of more power than the previously mentioned tubes.
Here is a nice example of a HB 6A5 amp that appeared on the Bottlehead website ion Monday:
The chassis is a steel broiling pan! The brass Magnequest trannies are used as phase splitters. The huge power trannie is a vintage UTC and is good for about 350V @ 400mA using a hybrid bridge.
/ed B in NH