NT57

The very early triode specifically deeveloped for radar application in GB: designed in 1936 by H.G.Hughes of Signal School. It had a short anode, to reduce its capacity and increase the frequency; the twin filament cathode granted 3.6 A emission. Derived from the NT46 radio transmitting tube, its electrode connection seals diverged to ensure better insulation at high impulse voltages, see the photo of the NT57D.
Used in the CH ground transmitters, in Type 79 shipboard early warning radar and in the early MB2 ground radar sets.
The NT57 evolved in the improved NT57A, then in the NT57D with simplified terminal seal process. In 1939 GEC and Signal School introduced the NT57T, using thoriated tungsten filament, with a relevant emission increase.

Before 1938 it was commonly accepted that only pure tungsten could be used as source of electrons in high voltage tubes.