Battery (vacuum tube)

In the early days of electronics, vacuum tube devices (such as radios) were powered by batteries. Each battery had a different designation depending on which vacuum tube element it was associated with.

Initially, the only such device was a diode with only a filament (cathode) and a plate (anode). Following the direction of electron flow, these electrodes were identified as "A" and "B", respectively and thus the associated batteries were referred to as the "A" battery and "B" battery, respectively. Later, when the control grid element was added to create the triode tube, it was logically assigned the letter "C" and supplied from a "C" battery. Subsequent addition of further internal elements to improve the performance of the triode did not require an extension to this series of batteries – these elements were either resistively-biased from the existing batteries, connected to ground or to the cathode.

This nomenclature was used primarily within North America. Different battery names were used elsewhere in the English speaking world.

An A battery is any battery used to provide power to the filament of a vacuum tube. It is sometimes colloquially referred to as a "wet battery". (A dry cell could be used for the purpose, but the ampere-hour capacity of dry cells was too low at the time to be of practical use in this service). The term comes from the days of valve (tube) radios when it was common practice to use a dry battery for the plate (anode) voltage and a rechargeable lead/acid "wet" battery for the filament voltage. (The filaments in vacuum tubes consumed much more current than the anodes, and so the "A" battery would drain much more rapidly than the "B" battery; therefore, using a rechargeable "A" battery in this role reduced the need for battery replacement. In contrast, a non-rechargeable "B" battery would need to be replaced relatively infrequently.)

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