"Jim Lux" wrote in message
...

snip

Where the term "ballast" came from, I don't know.

Possibly from the meaning "imparting stability." A ship's or boat's
ballast does that. Ballast around railroad ties does that. Ballast in a
hot air balloon does that.

A ballast tube in the filament circuit of a certain radio of my
acquaintance, the R-390A/URR, stabilizes the filament current for two
oscillator tubes, the VFO and the BFO.