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
  Electrical ballast
  An electrical ballast is a device intended to limit the amount of current in an electric circuit. A familiar and widely used example is the inductive ballast used in fluorescent lamps, to limit the current through the tube, which would otherwise rise to destructive levels due to the tube's...
  
  , barretter)
• B-- Diode
  Diode
  In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
  
   or other 2 element device (Includes TR/ATR tubes and spark gaps)
• C-- Triode
  Triode
  A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
  
  
• D-- Tetrode
  Tetrode
  A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.-Control grid:...
  
  
• E-- Pentode
  Pentode
  A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube , which was invented by the Dutchman Bernhard D.H. Tellegen in 1926...
  
   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 (klystron
  Klystron
  A klystron is a specialized linear-beam vacuum tube . Klystrons are used as amplifiers at microwave and radio frequencies to produce both low-power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for modern...
  
  s and inductive output tube
  Inductive output tube
  The inductive output tube or IOT is a variety of vacuum tube which evolved in the 1980s to meet increasing efficiency requirements for high-power RF amplifiers. The primary commercial use of IOTs is in UHF television transmitters, where they have mostly replaced klystrons because of their higher...
  
  s)
• L-- Vacuum capacitors
• N-- Crystal rectifier
  Rectifier
  A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
  
  s (This designation lived on as the "N" in EIA solid state device numbers like 2N3904)
• P-- Photosensitive types (phototube
  Phototube
  A phototube is a type of gas-filled or vacuum tube that is extremely sensitive to light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum.-Operating principles:...
  
  s, PMTs
  Photomultiplier
  Photomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...
  
  , camera tubes
  Video camera tube
  In older video cameras, before the mid to late 1980s, a video camera tube or pickup tube was used instead of a charge-coupled device for converting an optical image into an electrical signal. Several types were in use from the 1930s to the 1980s...
  
  , image converters)
• Q-- Resonant vacuum cavities
• R-- Ignitron
  Ignitron
  An ignitron is a type of controlled rectifier dating from the 1930s. Invented by Joseph Slepian while employed by Westinghouse, Westinghouse was the original manufacturer and owned trademark rights to the name "Ignitron"....
  
  s and mercury arc rectifiers
• S-- Vacuum switches
• T-- Storage, radial beam, and deflection control tubes (no known examples assigned)

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.

Oddities

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.

Famous Types

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:

• 1N23--Silicon point contact diode used in early radar mixers.
• 1P25--Infrared image converter used in WW2 night vision
  Night vision
  Night vision is the ability to see in low light conditions. Whether by biological or technological means, night vision is made possible by a combination of two approaches: sufficient spectral range, and sufficient intensity range...
  
   "sniperscopes".
• 2C39--"Oilcan" type planar triode.
• 2C43--"Lighthouse" type planar triode.
• 2D21--Miniature glass tetrode thyratron used in jukeboxes and computer equipment.
• 2P23--Early image orthicon TV camera tube.
• 3B28--Xenon half wave rectifier--ruggedized replacement for mercury vapor type 866.
• 3E29--Dual beam power tube used in radar equipment--a pulse rated variant of the earlier 829B.
• 4D21--VHF beam tetrode better known by Eimac
  Eimac
  Eimac is a trade mark of Eimac Products, part of the Microwave Power Products Division of Communications & Power Industries. It produces vacuum tubes for radio frequency applications such as broadcast and radar transmitters.-History:...
  
   commercial number 4-125A.
• 5C22--Hydrogen thyratron
  Thyratron
  A thyratron is a type of gas filled tube used as a high energy electrical switch and controlled rectifier. Triode, tetrode and pentode variations of the thyratron have been manufactured in the past, though most are of the triode design...
  
  for radar modulators.
• 6C21--Triode radar modulator for "hard tube" pulsers.
• 8D21--Internally water cooled dual tetrode used in early VHF TV transmitters.

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.