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 electronics. Many industrial, military or otherwise professional tubes were also produced. Only a few types are still used today, mainly in high-power, high-frequency applications and also in boutique guitar amplifiers.

Receiving tubes have heaters or filaments intended for direct battery operation, parallel operation off a dedicated winding on a supply transformer, or series string operation on transformer-less sets. High-power RF power tubes are directly heated. The heater voltage must be much smaller than the signal voltage on the grid and is therefore in the 5-25 V range, drawing up to hundreds of amperes from a suitable heater transformer. In some valve part number series, the voltage class of the heater is given in the part number, and a similar valve might be available with several different heater voltage ratings.

RETMA is the acronym for the Radio Electronic Television Manufacturers Association formed in 1953 - however the standard itself had already been in use since 1933, when RCA/Cunningham introduced the 1A6, 2A3, 2A5, etc.

Often designations that differed only in their initial numerals would be identical except for heater characteristics.

For examples see below

The system was used in 1942–44 and assigned numbers with the base form "1A21", and is therefore also referred to as the "1A21 system". The first numeric character indicated the filament/heater power rating, the second alphabetic character was a code for the function, and the last 2 digits were sequentially assigned, beginning with 21

For examples see below.

A four-digit system was maintained by JETEC since 1944, then by EIA since 1957 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.