4-250A transmitter tetrode
This transmitter tube, made by Eimac,USA, was widely used in amateur station finals or in modulator service. It is a coaxial tetrode with a direct heated thoriated tungsten filament (5 V, 14.5 A). Plate dissipation is 250 W max, the plate being made from hard metal with external high-temperature getter (zirconium coating?), thus the whitish colour of the plate. Plate voltage may get up to 4 kV, screen about 500 V. Due to short external connections, this tube can be used up to the 100 MHz-range.

It also is part of a series of transmitter tubes, the smallest being the 4-65A with 65 W plate dissipation, followed by the 4-125A, at the high-power side the 4-400A, 4-500A, 4-1000A. High-power types have graphite anodes. Equivalent European type is the QB 3,5/750.

Due to their short leads, these tubes mustn't be operated without a forced-air cooling - the air entering through a special socket, first cooling the base pins, then passing the tube inserted into a chimney and leaving the system by passing the top anode lead, which must carry some kind of cooling fin. Tube producers delivered suitable fins (like the one shown in the photo, at sometimes excessive prices), but selfmade fins did the same job as well - see page 4-400A.