In the years 1942-1944, the Radio Manufacturers Association used a descriptive nomenclature system for industrial, transmitting, and special-purpose vacuum tubes. The numbering scheme was distinct from both the numbering schemes used for standard receiving tubes, and the existing transmitting tube numbering systems used previously, such as the "800 series" numbers originated by RCA and adopted by many others.
The system assigned numbers with the base form "1A21", and this numbering scheme is occasionally referred to by tube collectors and historians as the "1A21 system".
The first digit of the type number was 1-9, providing a rough indication of the filament/heater power rating (and therefore the overall power handling capabilities) of the tube. 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. Multiple section tubes (like the 3E29 or 8D21) are assigned a letter corresponding to ONE set of electrodes.
Like all tube numbering systems, there are many inconsistencies between theory and practice. For example, there is no assigned letter code for cathode-ray tubes. 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 2F21 is not an actual hexode, but a pattern generating monoscope tube. Some very exotic types received generic designators, even when there was a more appropriate designator available. 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.
The first-digit filament/heater power rating confusingly gathers valves of widely-differing ratings. The 2G21 is a subminiature Triode-Hexode, with a maximum anode (plate) current of some 0.2 milliamps and a maximum voltage of 45 volts. The 2J42 Magnetron, with a power output of some 7 kilowatts, is rated for anode current of 4.5 amps (pulse peak) at anode voltage of 5,500 volts.
Many of the "1A21" series are well known to collectors and restorers of WW2 vintage radio equipment. A short list of well-known or historic types numbered under this system:
This numbering system was abandoned in 1944 in favor of a non-descriptive numbering system of 4 digit numbers beginning with 5500. This new system persisted until the final days of tubes, with type numbers registered up into the 9000 series.