Definition
In vacuum-tube electronics, the B-battery is the power source that supplies the high positive DC voltage to the plate (anode) of a vacuum tube, allowing electrons to flow from the cathode to the plate and producing the tube's amplifying or signal-generating action.
Plain English
The B-battery is the battery that powers the main working voltage of a vacuum tube. Without it, the tube cannot do its job.
Context Anchor
Seen mainly in older aircraft maintenance texts, vintage radio equipment, and historical electrical-system discussions.
Derivation
Early vacuum-tube circuits used three separate power sources, labeled by letter. The A-battery heated the filament, the B-battery supplied the plate voltage, and the C-battery set the grid bias. The 'B' is simply the second letter in that lettering scheme; the labels stuck even after the actual batteries were replaced by power supplies.
Why Pilots Care
Correct B-battery voltage is required for reliable operation of older radios; low or missing voltage prevents transmission and reception.
Intuition Check
Do not read B-battery as “battery number two” or a backup aircraft battery. In this term, B-battery means the high-voltage battery used by older vacuum-tube radio equipment.
Example Sentence 1
The mechanic restoring the vintage trainer's radio noted that the original B-battery had long since been replaced by a modern DC power supply delivering the same plate voltage.
Example Sentence 2
Many pre-1960s aircraft carried separate A-batteries and B-batteries to power their tube radios.