A battery

A battery, in tube electronics, supplies direct current to heat the filament or cathode heater inside a vacuum tube. This is distinct from the high-voltage anode battery that accelerates electrons toward the plate. The heating filament must glow hot enough to emit electrons through thermionic emission, typically requiring 1.5 to 12 volts depending on tube design. Without adequate heater current, the tube produces no amplification or detection.

Filament batteries were usually low-voltage cells connected in series to reach the needed voltage. Common choices were lead-acid accumulator batteries, dry cells, or storage batteries rated for continuous draw. As radio and laboratory equipment became portable in the early twentieth century, the heater battery determined much of a device's weight and size. A tube requiring 6 volts might use four 1.5-volt cells in series; a 12-volt tube needed eight cells or a small lead-acid accumulator.

Voltage stability and current demands

The heater must be supplied at rated voltage to function correctly. Running the filament too cool reduces emission and cuts output; running it too hot shortens tube life dramatically. A filament battery that sagged under load, or one that drifted over time, caused performance to drift with it. Older portable radios and test equipment included a heater ammeter on the panel so operators could monitor filament current and detect battery aging.

The term fell from common use as AC line-operated power supplies became standard in stationary equipment, and later as miniaturized electronics eliminated large vacuum tubes altogether. In modern contexts, "battery" in tube work refers to the anode or plate battery instead. The heater supply is now typically a secondary winding on the power transformer, regulated by silicon rectifiers rather than supplied by a separate battery pack.