The RK-20A is a 40 watt dissipation pentode made by Raytheon with a maximum frequency of 30 MHz for full output. This tube is supposed to be the same as the CK-70 and is an improved RK-20. The first example has a white "isolantite" base, but some tubes of this type had brown baklite base as in the second example. The illustration in the 1938 Raytheon Handbook of the RK-20A shows a small ceramic insulator holding the filament springs and grid as is shown in the first example, but the second example has a much larger ceramic spacer element support that extends to the four plate support wires. The third example must be the most recent as it has lost the large plate cap structure and upper arbor press and appears less rugged than the early tubes. It also has a carbon anode with two positioning ceramic insulators and mica stabilizing sheets. The fourth example appears to be the same as the second, but is marked on the glass and the carton as an 804/RK-20A. The RK-20A isn't supposed to handle 1500 volts and has a higher amperage filament than an 804. The RK-46 is the same tube with a 12.6 volt, 2.5 amp filament and the RK-47 is a beam tetrode version.
Length = 8.50" Diameter = 1.88"
Max voltage = 1250
Max current = 92 ma
Fil voltage = 7.5
Fil current = 3.25 amps