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:
- 1-- No filament/heater, or cold cathode device
- 2-- Up to 10 W
- 3-- 10-20 W
- 4-- 20-50 W
- 5-- 50-100 W
- 6-- 100-200 W
- 7-- 200-500 W
- 8-- 500W-1 kW
- 9-- More than 1 kW
The second character was a letter broadly identifying the class of tube:
- A-- Single element (ballast, barretter)
- B-- Two-element device such as:
- Diode
- Transmit/receive tube (TR cell), cold-cathode water vapor discharge tube for use in radar systems, shorts the receiver input to protect it while the transmitter operates
- Anti-transmit/receive tube (ATR cell), cold-cathode water vapor discharge tube for use in radar systems, decouples the transmitter from the antenna while not operating, to prevent it from wasting received energy
- Spark gap
- C-- Triode
- D-- Tetrode
- E-- Pentode or beam power tetrode
- F-- Hexode
- G-- Heptode
- H-- Octode
- J-- Magnetically controlled types, usually incorporating a resonator (essentially, magnetrons)
- K-- Electrostatically controlled types, including a resonator (klystrons and inductive output tubes)
- L-- Vacuum capacitors
- N-- Crystal rectifiers (This designation lived on as the "N" in the EIA/JEDEC EIA-370 solid state device numbers standard, like 2N2222)
- P-- Photosensitive types (phototubes, photomultipliers, camera tubes, image converters)
- Q-- Resonant vacuum cavities
- R-- Ignitrons and mercury arc rectifiers
- S-- Vacuum switches
- T Storage, radial beam, and deflection control tubes (no known examples assigned)
- V-- Flash tubes
- W-- Travelling wave tube
- X-- X-ray tube
- Y-- Thermionic converter
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.