I've not been into R/C stuff, but I am into 'submini' tubes.
I recently built a 5-tube regenerative receiver for the 49 meter band using
5678's for the 2 stage tuned RF amp, detector, and first audio amp. A 5676
serves as the final audio driver.
Performance is awesome, and it runs on batteries. Warm-up time is at most
3 seconds, nearly instant-on operation.
These tubes are only about 1 cm in diameter, and perhaps 4 cm long, with
wire leads (no pins, no socket needed).
This is my fourth tube based radio. A friend was joking that these
recreations
sould be known as 'bunker buddies' 1 thru 4, as they use no semiconductors
at all.