Marconi-Osram tube designation

This is a list of vacuum tubes or thermionic valves, and low-pressure gas-filled tubes, or discharge tubes. Before the advent of semiconductor devices, thousands of tube types were used in consumer and industrial electronics; today only a few types are still used in specialized applications.

Vacuum tubes fall into three mainly non-interchangeable categories regarding their heater or filament voltage (some tubes' heaters run at a voltage and current suitable for either series or parallel operation, e.g., 6.3 V at 300 mA).

The currents drawn by parallel-connected types, and the voltages across series-connected types, vary widely according to the tube's heating power requirements. Otherwise identical tubes were manufactured in several variants with different heater characteristics (but usually the same power, e.g. 6.3 V/300 mA and 12.6 V/150 mA). See, for example, the RCA Receiving Tube Manuals.

In half-indirectly heated tubes the cathode and one side of the filament share the same pin.

The system assigned numbers with the base form "1A21", and is therefore also referred to as the "1A21 system".

First digit Filament/heater power rating:

Next character: Function:

The last 2 digits were sequentially 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.

Examples

RETMA is the acronym for the Radio Electronic Television Manufacturers Association, originally the RMA, later RTMA, then EIA (Electronic Industries Association, since 1997 Electronic Industries Alliance).

Often, but not always, vacuum tube designations that differed only in their initial numerals would be identical except for heater characteristics. Exceptions include: the 12BR7 and 9BR7 are unrelated to the 6BR7; the 4BL8/XCF80 is the 4.6 Volt (600mA series heater) version of the 6BL8/ECF80, but the 450mA series heater version is the 6LN8/LCF80 rather than 6BL8. The change of letters was required as the nominal heater voltage for both types is '6' because the 6LN8 is 6.0 volts as opposed to the 6BL8 which is 6.3 volts.

For examples see below

A four-digit system was maintained by the EIA for special industrial, military and professional vacuum and gas-filled tubes, and all sorts of other devices requiring to be sealed off against the external atmosphere.

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