Tube of the Month – The 30

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.