Another small directly heated triode that surely deserves more attention than it gets. The 30 was designed as both detector and amplifier tube for the portable radio receivers. It is a battery tube like the 31.
As it was designed for battery powered equipment, oxide coated filament was made in such way that can operate at as little power as possible; with only needed 2V filament voltage and 60mA to operate!
It could be a perfect companion for the 31 in a very low powered SET amp designated to drive ultra efficient speakers, sole tweeter amp in active powered speaker, or even in a small all DHT headphone amp. The plate curves show the typical excellent linearity of directly heated triodes.
Article: link.