A cathode in a vacuum tube that is heated by a filament which is not a part of the cathode. The cathode is made in the form of a metal tube whose surface is coated with a material that emits electrons when it is heated. The filament, a small resistance-wire heater inside the cathode, heats it but does not touch it. Indirectly heated cathodes allow the filament to be heated with alternating current without causing the hum that would be present if the AC passed through the cathode itself.