The 6C21 is a 300 watt triode with a mu of 30 designed for pulse modulator service. As an amplifier, the dissipation becomes 450 watts and it will operate at full power up to 40 MHz. The first example is an EIMAC and the second is a later EIMAC with a different shaped envelope. The third is a Machlett, the fourth a GE and the fifth and sixth are different styles of Westinghouse.
Length = 12.63" Diameter = 5.13"
Max voltage = 30,000 pulse; 6000 amplifier
Max current = 15 amps pulse; 600 ma amplifier
Fil voltage = 8.2 pulse service; 7.5 amplifier
Fil current = 15.9 to 17.7 amps; 15 amps amplifier