Mullard–Philips tube designation

In Europe, the principal method of numbering vacuum tubes ("thermionic valves") was the nomenclature used by the Philips company and its subsidiaries Mullard in the UK, Valvo in Germany, and Dario in France, from 1934. This system allocated meaningful codes to tubes based on their function. This European common code system became the starting point for the Pro Electron naming scheme for active devices (including tubes and transistors).

The system allowed for cross-referencing with the American RETMA tube designation, the Marconi-Osram tube designation, and with military numbering systems such as common valve (CV) numbering in the United Kingdom and the Joint Army–Navy (JAN) tube designation in the US.

European tube manufacturers agreed on the system, but in the UK, MOV (Marconi-Osram Valve), STC/Brimar and Mazda/Ediswan maintained their own systems. Most MOV tubes were cross-licensed copies of RCA types, with a British designation. For example, an MOV X63 valve was the same as an RCA 6A8 tube. Brimar, which stood for "British Manufactured American Radio" (valves), used all American designations. STC/Brimar was a UK subsidiary of the American giant ITT (International Telephone and Telegraph).

Special quality tubes were sometimes identified by placing the numerical part of the designation between the heater rating and the tube type. The special quality could be anything, from rugged designs for military and industrial use, through devices with exceptionally low noise and microphony, to designs primarily optimised for long life without cathode poisoning when used for switching in a digital computer (but not necessarily with exceptional qualities as an amplifier). For example, an ECC81 manufactured as a special quality (SQ) tube would usually be designated 'E81CC'. The system was not universal as other special quality designation systems existed. An EF91 in its special quality version was designated 'M8083' (the 'M' meant Military) as in this case the standard EF91 was derived from the M8083 military design. Also, the SQ tube was not always designed for the same tasks or given the same Maximum Ratings (for example the E80F was more suitable for audio and electrometer applications, lacking the RF screening of the EF80, with anode and screen grid power ratings roughly half the EF80.

...
Wikipedia