An electron tube, also known as a valve, is an evacuated glass or metal tube which may contain gas at low pressure, through which electrons flow between two or more electrodes. A heated filament, the cathode, emits electrons which are attracted to the positively-charged anode.
The diode, used for rectification, consists of a negative cathode, which emits electrons when heated, and a positive anode or plate. The triode, used for amplification, has a perforated control grid between the cathode and the anode; a signal fed to the grid provides an amplified signal at the anode.
Electron tubes have been largely replaced by transistors and other semiconductor devices.