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Each different type of tube received an identifier, or designator, for example 12AX7, EABC80 or 300B. Some kind of identification was obviously essential, as for any electronic part, just so that users can order the parts they need. The tube identifiers were generally independent of any particular manufacturer, so the corresponding tube could frequently be obtained from different sources.
There were two main systems in use: Philips in Europe, giving codes like ECC83; and RETMA in the US, giving designators like 6SN7.
Philips Coding
Despite the name, this coding was used by nearly all European manufacturers, with Mazda as the principal exception. It was invented and maintained by Philips and Mullard.
The first letter specifies the filament requirements. By far the most common letter is E, meaning 6.3V. Televisions and older AC/DC radios didn't have a fixed 6.3V filament supply. Instead, they wired all the heaters in series across the AC power. That meant they had to use the same current, generally 300 mA for televisions and 100 mA for radios, rather than the same voltage. Philips encoded fixed-current filaments specially. Commonly-used codes were as follows:
| D | 1.5V for battery operated radios | |
| E | 6.3V | |
| G | 5V for B+ rectifiers | |
| O | cold-acthode tubes, e.g. voltage regulators | |
| P | 300 mA series connected, for televisions. If that happened to result in a 6.3V filament, like the ECC8x twin-triodes, E was used rather than P | |
| U | 100 mA series connected, for AC/DC radios |
There were many other assigned codes (for example, for 2V battery operated radios), but by 1950 they were all obsolete.
The second and subsequent letters described the elements of the tube, as follows:
| A | signal diode | |
| B | twin signal diode with shared cathode | |
| C | small signal triode | |
| D | power triode (rarely used) | |
| F | pentode | |
| H | hexode frequency changer | |
| K | heptode or octode frequency changer | |
| L | beam tetrode | |
| M | magic eye | |
| Q | nonode (only one, the EQ80) | |
| Y | EHT rectifier | |
| Z | B+ rectifier |
These could be combined as required. So 'CC' was a twin triode, 'CF" was a triode plus pentode, and 'ABC' was a triode combined with a single diode and a dual diode (used in FM demodulators).
The first digit described the base:
| None | Philips side-contact | |
| 1 | meant "look at the second digit", e.g. EF180 | |
| 3 | octal | |
| 4 | 8-pin Rimlock | |
| 5 | miscellaneuous - anything not covered elsewhere | |
| 7 | Loctal (little used since Loctal tubes were rarely used in Europe) | |
| 8 | B9A 9-pin | |
| 9 | B7G 7-pin |
Meanings were assigned to other values but were rarely used.
Remaining digits were just used to identify specific types, and had no meaning.
Some examples:
- ECC83: 6.3V twin triode with B9A base
- UCH81: 100 mA triode-hexode frequency changer with B9A base
- EC90: 6.3V single triode with B7G base
- PL500: 300 mA power pentode with large 9-pin base
- ECC33: 6.3V twin triode with Octal base (near-equivalent to the 6SN7)
- GZ40: 5V full-wave rectifier with 8-pin Rimlock base
US Coding (RETMA)
The US codes give less information about the tube than the Philips code.
The first number is the filament voltage. '6' means 6.3V. There is one exception: '7' normally means a 6.3V heater, but with a Loctal 8-pin base - except for a tiny number of tubes that really did have 7V heaters.
The letters had no meaning. They were assigned serially, in general, as required.
The last digit is the number of distinct "elements" in the tube. This is a count of the electrodes, with the filament counting as a single element. So a 6SN7 has a 6.3V filament, two triodes of three elements each, and the filament, giving a total of 7.
Any letters after that describe the specific envelope.
- Originally the absence of a letter meant a metal outer envelope, like the 6CA7, but for miniature tubes it meant a conventional tubular glass envelope, e.g. 12AX7
- G' meant a coke-bottle style envelope, like the 6AS7G.
- 'GT' meant a squat, tubular glass envelope, like the 6SN7GT
- 'A", 'B' or 'C' meant improved versions., so a 6SN7GTA was an improved version of the 6SN7.
There was no special treatment for constant-current tubes, which were designated by whatever voltage happened to correspond to the required current. For example, the 27BG5 was the equivalent of the PL500, a 300 mA power pentode that happened to require 27V.
Other Codings
- Special tubes carried a 4-digit number in order of their introduction, for example the 5692 which was a special quality version of the 6SN7
- Western Electric had their own coding, generally a 3-digit sequential number possibly followed by a letter, e.g. 300B, 417A
- The UK company Mazda used the Marconi-Osram coding, e.g. the KT88 beam tetrode
- Very old tubes carried whatever number their original manufacturer gave them, e.g. 76
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