For a little fun..
The directly heated 2E24 is a somewhat neglected gem, perhaps due to its directly heated cathode. It uses a center tapped filament rated
6.3V@0.65A. It has a 10W CCS, 13.5W ICAS plate dissipation. The filament heating time is specified as <2 seconds. It seems to be the quick-heating version of the more popular indirectly heated 2E26.
It was designed for mobile and emergency communications gear and would be interesting where energy economy is wanted as the filament would not have to be kept hot during standby. The 2E24 is useful as a modulator and an RF amplifier and should be a great "QRP" tube where the rest of the set is solid state.
The AB2 data given is for audio service, but with there is no reason it could not be used as a linear RF amplifier. A pair of them can make 42W with 0.43W of drive, -15V bias, 400V on the plates, and only 125V on the screen. Resting and maximum signal plate current for a push pull pair are an efficient 18 and 150mA. For linear RF service, a slightly higher idling current might be wanted to improve linearity at low signal levels.
Plate-modulated RF ratings allow a 13.5W carrier CCS and 18W ICAS per tube. For CW, 20W CCS and 27W ICAS are available. Drive is about 0.2W and bias is about -50V. The full ratings apply below 125MHz.
The 2E24 is very inexpensive, perhaps because it was made in quantity for mobile use, the product life cycle of that equipment was fairly short compared to tube consumption, and the need for supplying grid current in order to get useful power from it keeps most audio hobbyists disinterested. It can be found for $4 on the internet.
Does anyone know of equipment that uses the 2E24?