I am frequently asked to explain what all of the “letters and numbers” mean in a vacuum tube number. In the early days of radio, tube numbers were haphazardly assigned to new tube types. As more vacuum tube types were developed, it became more difficult for technicians to recognize any fundamental characteristics about a tube. To alleviate this confusion, the Radio Manufacturers Association (RMA) developed a tube numbering standard. The standard allows a technician to have at least a basic understanding of the construction and purpose of a tube.
Notes: Early vacuum tubes with 2-digits (such as #50) and 3-digits (such as #485) predate the RMA standard. Similarly, 4-digit industrial/commercial numbers are not part of this standard. The standard consists of a first numeral(s), followed by a letter or two letters, a last numeral, and optionally letter suffix(es).
Also, there are tube numbers that look like they were part of the numbering standard, but in fact they were not. An example is 2D21.
I have compiled the following information from several vintage books and publications in my personal collection. 
For a general primer on the subject, [ here is a PDF ] from NATIONAL RADIO-TV NEWS, April-May 1953 issue, that explains some of the basics. I scanned the article and cleaned it up with photoshop for your convenience.
I. the FIRST numeral(s)
The first numeral(s) basically refers to the filament/heater voltage, rounded down. Cold-cathode tubes have “0” designation.
- All tubes with 2.0 filament voltage or lower have “1” designation.
- many Loctal tubes were given the “7” leading numeral for 6.3 volt tubes and “14” numeral for 12.6 volt tubes.
- 2A3 has filament voltage of 2.5 volts. 2.5 was rounded down to 2, thereby the ‘2’ in 2A3.
- 1LA6 has 1.4 volt filament, which rounded down, explains the “1” designation.
- 1A6 has 2.0 filament voltage and was also assigned the “1” numeral.
- 7N7 is a 6.3 Loctal tube; 14A4 is a 12.6 volt Loctal tube.
- 0A2 is a cold cathode voltage regular tube
II. the Letter(s) between the numerals
The one letter or two letters between the numerals serve to distinguish between tubes that have the same number of elements and same filament voltage. This primary letter is called the “serial letter“, and if the tube has two letters, the “serial letter” is the 2nd letter.
- Rectifiers were assigned starting with Z and working backward through the alphabet. (example: 5Z3)
- All other tubes were alphabetically A to Z. (example: 6A3)
When all single letters were used individually, newer designations were assigned a leading alphabetically letter, starting with A. After all “A” were exhausted, subsequent assignments would start with “B”, then “C”, etc.
- 6K6 existed, so the next developed tube with a 6.3 volt filament and 6 useful elements was given the 6AK6 designation, the next developed tube with a 6.3 volt filament and 6 useful elements was given the 6BK6 designation
The leading “S” designation exception:
- The letter ‘S‘ before the serial letter designates “single-ended”. Single-ended in this context refers to the fact that older tube designs often had a control element, usually the control grid, exiting the tube on the top — the top cap terminal. Newer designs usually terminated all control elements at the base, and were appropriately called “single-ended” tubes and given the “S” designation.
- Example: 6K7 has a top cap, 6SK7 has only base pins.
- Do not confuse a suffix “S” with this single-ended designation. For example: tube 57S would not qualify.
III. the LAST numeral
The last number, which follows the “serial letter”, explains the number of “useful elements” inside the tube. “Useful elements” are elements inside the tube that are brought out to a pin and can be used. The filament is always counted, and the suppressor is counted if it is brought out to a pin. If the shell is brought out to a base pin, or the tube design reserved a pin for the shell, it is also counted.
- 6L6 = the trailing “6” refers to the fact that a 6L6 has 6 useful elements: heater, plate, control grid, screen grid, cathode, suppressor.
- 2A5 = heater, plate, control grid, screen grid, cathode. It also has a suppressor, but the suppressor does not terminate at one of the tube pins, so it is not counted because the operator cannot “use” it.
Suffix letter(s) generally explain the physical properties (or differences) between tubes of the same type. Letters can be “stacked” to convey more than one piece of information to the technician.
Example: 5Y3WGTA conveys a ruggedized US military issue tube with a small glass envelope and newer design than original 5Y3G.
Of these suffixes, the most useful to memorize (in my opinion) are:
- G = Glass envelope, which distinguishes from a metal-envelope tube
- W = Ruggedized construction, meets “MIL-1-A” US Military specifications, and therefore most W tubes are US military inventory.
A more complete list of suffixes and their exact meaning:
- (None) = Without more context, a tube with no suffix does not tell you very much. If you know that both metal and glass versions were manufactured, then no suffix is a METAL ENVELOPE tube (example: original 6L6, 6F6, 6V6, 6N6, etc… were all metal envelope tubes). If only one type of envelope was manufactured for that type, then no suffix simply refers to the tube being the original design of the tube. (Example: 2A3, 6A3).
- A = 2nd generation of same tube type with a small improvement or higher ratings than original design, but not sufficiently different to warrant an entirely new designation. Backwards compatible with earlier designs. The ‘A’ improvements were primarily “controlled heater warm-up time” and/or “low-noise-low-hum.”
- B = 3rd generation of same tube type with a small improvement or higher ratings than earlier designs, but not sufficiently different to warrant an entirely new designation. Backwards compatible with earlier designs.
- C = 4th generation of same tube type with a small improvement or higher ratings than earlier designs, but not sufficiently different to warrant an entirely new designation. Backwards compatible with earlier designs.
- G = Glass envelope and Octal base. Distinguishes from a metal-envelope tube.
- GB = T-5½ glass envelope
- GL = T9 glass envelope with Loctal base
- GM or MG = Metal-coated glass envelope with Octal base
- LM = MT-8 metal envelope and octalox base
- LT = T9 glass envelope and octalox base
- M = Metal envelope and Octal base
- ML = T9 metal envelope with Loctal base
- M-R = while not technically a suffix, “MR” when stamped on tube base only means that the tube was manufactured during WW2 for “Maintenance and Repair” of civilian radios. (During WW2, military need for vacuum tubes created a large shortage of tubes that could be purchased by civilians.) M-R has absolutely no special “audiophile” connotation.
- S= Spray shield coating on a glass envelope tube
- T = short glass envelope
- W = Ruggedized construction, meets “MIL-1-A” US Military specifications
- Y = Micanol base
Summary and Conclusion
The RMA tube numbering code was certainly an improvement from the random numbering scheme that preceded it. You might summarize the information as:
- You have a good idea of the filament voltage
- The Serial Letter, in combination with the Last Numeral, usually tells you whether the tube is a rectifier or something else. A high-alphabetical serial letter (T or higher) and a low last numeral (5 or lower) is a good indication that the tube is probably a rectifier. (Examples: 5Z3, 6W4, 6AX5GT, 5AS4A). Unfortunately, there are exceptions (example: 6CA4 is a rectifier).
- Suffix letters do a reasonable job of conveying physical traits
- A. Ghirardi, “Radio Troubleshooter’s Handbook, 3rd Edition”, 1943, pages 588-589.
- “How Radio and Electronic Tubes Work”, Volume 8FR-1, National Radio Institute, 1949, pages 34-36.