Interpreting the Type Numbers
In spite of the fact that a large number of American 'Lend-Lease' valves are now available in this country, it seems that many wireless users do not appreciate the meaning of the numbers allotted to the various valve types. Most current American receiving valves are designated according to an system standardised by the American RMA in 1933. The type number is divisible into four groups having the significance shown in the sketch.
The figures indicates the actual heater voltage, ignoring fractions of a Volt, except in the case of 2 Volt valves, where 1 is used for all voltages below two and 2 is used for voltages between 2.1 and 2.9 Volts. This convention originated from the necessity of distinguishing between the 2.0 and 2.5 Volt series.
Type letters were assigned to valves as they were introduced, starting at A for any type except rectifiers, and from Z working backwards for rectifiers. Double letters are new being used for many new valves. It the first letter of a two-letter group is 'S' it indicates the single-ended version of an existing type. Thus the 6SJ7 is similar to the 6J7 but its working grid is brought out to a pin in the base instead of to a top cap.
In earlier types this number referred to the number of useful elements; thus at battery triode would be indicated by 3 (filament, grid and anode), but in later types it relates to the number of external contacts - two for heater. one for cathode, etc.
This gives the type of envelope and base thus:-
- M: metal envelope, octal base
- G: glass envelope, octal base
- GM: metal-coated glass envelope, octal base
- GT: short glass envelope, octal base
- GL: integral glass envelope and loktal base
- ML: integral metal envelope and loktal base
- LM: metal envelope and loctal base
- LT: glass envelope and loktal base
- GB: integral glass envelope and base.