Vacuum tube battery

In the early days of electronics, devices that used vacuum tubes (called valves in British contexts), such as radios, were powered by batteries.

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.

The filament is primarily a heat source and therefore the "A" battery supplies significant current and rapidly discharges.

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 in American schematics.

Until the early 1930s this was common practice in valve (tube) radio sets but was largely superseded by grid leak resistors or voltage divider biasing.

In more recent times, they were popular in schools and colleges as a convenient variable voltage source in science classes.

The most popular battery is the 9-volt type with taps every 1+1⁄2 volts that accept banana plugs.

They were cumbersome, needed frequent recharging, and leaked battery acid, reducing the wife approval factor in the home.

Philco and other companies sold popular battery eliminators that plugged into light sockets.