Main article: 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 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. Tube bases and envelopes [ ].

Main article: Abbreviations used in this list [ ] • ST – Shouldered tube • GT – Glass tube • MT – Miniature tube, such as or • FL – Subminiature all-glass elliptical body and flat bases with long, inline 'flying leads' (pigtails) that are soldered into the circuit • SL – Subminiature all-glass elliptical body and flat bases with short inline leads that can be soldered or can be mated with a special socket. (Flying leads can be cut short to fit into inline sockets.) • R8 – Subminiature all-glass round body and base with 8 flying leads or stiff pins arranged in a circle Numbering systems [ ] North American systems [ ] RMA system (1942) [ ]. Main article: The system assigned numbers with the base form '1A21', and is therefore also referred to as the '1A21 system'. Main article: RETMA is the acronym for the Radio Electronic Television Manufacturers Association formed in 1953. • The first character group is a number representing the heater voltage rounded to the nearest whole number; 0 indicates a. • One or two letters assigned to the devices in order of development. • A single numeral that represents the number of active elements in the tube.

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