B battery

A B battery is a dry cell or battery pack designed to supply the positive plate voltage (anode voltage) required by vacuum tubes in electronic circuits. While A batteries supplied the filament heater current, B batteries provided the much higher voltages needed at the tube's plate to accelerate electrons from cathode to anode, typically ranging from 45 volts to over 300 volts depending on the tube type and application.

B batteries were built from stacks of individual dry cells connected in series. A small radio receiver might use a 45-volt B battery made from 30 small cells; larger applications like public address amplifiers or early transmitters could require 300 to 500 volts from multiple battery packs wired together. The cells were housed in cardboard sleeves with metal terminals for connection, and tap points along the stack allowed circuits to draw different voltage levels for different tube stages.

Why B batteries became obsolete

The B battery was the most expensive component in portable tube equipment and depleted rapidly under load. A typical 45-volt B battery in a radio would last only 10 to 40 hours of continuous use depending on the tube current draw. This made B batteries impractical for anything but light-duty portable work. By the 1940s, AC-powered tube equipment with transformer-based power supplies became standard, and battery-powered radios began switching to single-tube designs with lower plate voltages or to permanent magnet loudspeakers that could work with lower voltages.

The term B battery survives in tube radio documentation and restoration work. Modern vacuum tube applications, whether in guitar amplifiers or vintage radio restoration, use solid-state regulated power supplies instead, making the old B battery a relic of early twentieth-century portable electronics.