The RK-20 is a 40 watt dissipation pentode with a maximum frequency for full output of approximately 30 MHz. It was a popular oscillator and amplifier tube in amateur radio transmitters that was first introduced in 1934. It would operate as a screen modulated crystal oscillator. They came in at least 3 versions. The type with the smaller plate cap, the cross shaped upper press and the filament springs protected in mesh, may be the original (Radio, August 1934 and ARRL Handbook, 1936). The second type has a similar press but with a larger cap and the filament springs are exposed above the mesh. It also has four small insulated extensions from the lower skirt of the plate for stability. The third type has a simple flat upper press and the larger cap. The plate supports are not incorporated in the press, but to a clamp above the press. It has the same plate construction as seen on the RK-20A of 1937/8 (Raytheon Handbook). RCA produced a similar tube, the 804 starting in 1936 and Raytheon made RK-20s or RK-20As with 3 amp filaments marked 804 to fill that market. The RK-20 had lower capacities which would allow it to operate at full power at higher frequencies than the 804.
Length = 7.75" Diameter = 2.06"
Max voltage = 1000 (1250 in 1936)
Max current = 85 ma (92 ma in 1936)
Fil voltage = 7.5
Fil current = 3 amps