The type 45 vacuum tube was a power amplifier triode introduced in March 1929 by the Radio Corporation of America (RCA). It was introduced as the UX-245 under the old style of tube numbering. Within a year or two of its release, it was renumbered "45", under the new designation system of dropping everything but the last two digits. At the same time, its bulb shape was changed from the large globe style to the smaller "ST" shape.
At the end of the 1920s, alternating current had almost completely supplanted batteries as the main power source for radio receivers. There were other power triodes available, such as the UX-171A, UX-250, and the UX-210; however, the UX-171A did not provide enough output for the new multi-tube circuits, and the UX-210 and UX-250 were usually too expensive for home receivers. Also, the 45's 2.5-volt filament fit in nicely with the other 2.5-volt heater tubes shortly to be introduced.
Radio engineers wanted a tube that could provide adequate audio output, while keeping costs to a minimum. The UX-245 fit the requirements nicely, and its popularity was almost assured from its introduction. Operated at its published conditions, one 45 can provide two watts of audio output, quite a step up from the UX-171A's 0.79 watts. Two type 45s operated in a push-pull circuit can supply nearly six watts.
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