B battery (vacuum tubes)

In electronics, a B battery is any battery used to provide the plate voltage of a vacuum tube. It is sometimes colloquially referred to as a "dry battery" (although there's no reason why a "wet" battery of suitable voltage couldn't be utilised for the purpose) 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 A Battery serves primarily as a heat source and rapidly discharges, while the B battery experiences very little current draw and retains its stored capacity far longer than an A Battery.

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Even when the plate voltage rail is fed by a power supply rather than a battery, it is generally referred to as the "B+" line.

The alphabetic designation of these batteries is derived from the historic identification of the elements of the vacuum tube. Initially, the only such device was a diode with only a plate and cathode. Following the direction of electron flow, these electrodes were identified as "A" and "B" for the cathode and anode (plate), respectively. Later, when the control grid element was added to create the triode tube, it was logically assigned the letter "C."

Because plate voltages can be as high as 300v DC, multiple B batteries may be connected together in series to additively provide the required operating voltages.

Due to the much higher available voltage of B batteries, they must be handled more carefully than other battery types due to the ability of the battery to shock and/or burn the person handling the battery. With common flashlight cells, the normal resistance of the human body is far too high for much current to flow, but the shock hazard increases as the voltage increases.

See also

• A battery (vacuum tubes)
• C battery (vacuum tubes)