The KY21 is a high voltage, grid controlled, mercury vapor rectifier made by Eimac. It was announced in March 1938. The first example was a production unit while the second may be developmental as it only has 4 pins instead of 5. The 3rd example shows the more modern construction.

Length = 7.25" Diameter = 2.87"
Max voltage = 11000 PIV
Max current = 750 ma
Fil voltage = 2.5
Fil current = 10 amps         GOTO INDEX