RMA tube designation

In the years 1942-1944, the Radio Manufacturers Association used a descriptive nomenclature system for industrial, transmitting, and special-purpose vacuum tubes.

The assigned numbers were as follows: The second character was a letter broadly identifying the class of tube: The last 2 digits were serially assigned, beginning with 21 to avoid possible confusion with receiving tubes or CRT phosphor designations.

Some unusual types received rather mundane sounding designations, based solely on electrode count, because there was no better place to put them.

For example, the 2H21 "phasitron" phase modulator tube used in early FM broadcast transmitters was assigned an "H" (octode) designator, when it would have been a perfect candidate for the otherwise unused "T" category for deflection controlled tubes.

This new system persisted until the final days of tubes, with type numbers registered up into the 9000 series.