In electronics, a vacuum tube, electron tube (in North America), thermionic valve, tube or valve is a device controlling electric current through a vacuum in a sealed container. The container is often thin transparent glass in a roughly cylindrical shape. Within the glass is a vacuum with a cathode and anode at either end. Electrons flow from the cathode to the anode through the vacuum, a demonstration of the Edison effect. The introduction of a grid between the cathode and anode makes it possible to amplify a current. A negative voltage applied to the grid will subdue the current traveling between cathode and anode through the vacuum by a larger value than the current applied to the grid. Hence the grid may be used to modulate the current through the tube.
Vacuum tubes are used for rectification, amplification, switching, or similar processing or creation of electrical signals. Tubes rely on thermionic emission of electrons from a hot filament or hot cathode. Electrons travel to the anode (or plate) when it is at a positive voltage with respect to the cathode. Additional electrodes between the cathode and anode regulate current, allowing a tube to amplify or to switch.
Tubes were critical to the development of electronic technology, which drove the expansion and commercialization of radio broadcasting, television, radar, sound reinforcement, sound recording and reproduction, large telephone networks, analog and digital computers, and industrial process control. Although some applications had counterparts using earlier technologies such as the spark gap transmitter or mechanical computers, it was the invention of the triode vacuum tube and its capability of electronic amplification that made these technologies widespread and practical.
In most applications, solid-state devices such as transistors and semiconductor devices have replaced tubes. Solid-state devices last longer and are smaller, more efficient, more reliable, and cheaper than tubes. Tubes can be fragile, sometimes generate significant unwanted heat, and can take many seconds—many minutes in critical applications—after powering on to warm to a temperature where they perform within operational tolerance. However, tubes still find uses where solid-state devices have not been developed, are impractical, or where a tube has superior performance, as with some devices in professional audio and high-power radio transmitters. Tubes are still produced for such applications.