The Evolution of The Radio Valve
In the application of electronics, a vacuum tube, electron tube in North America, or valve is a device for controlling the flow of electric current through a vacuum in a sealed container. The container is often thin transparent glass in a cylindrical shape.
The simplest vacuum tube, the diode, is essentially an incandescent light bulb with an added electrode inside. When the bulb's filament is heated white-hot, electrons are boiled off its surface and into the vacuum inside the bulb.
If the electrode, called a "plate" or "anode", is made more positive than the hot filament, a direct current flows through the vacuum to the electrode (a demonstration of the Edison effect). As the current only flows in one direction, it makes it possible to convert an alternating current (AC) applied to the filament to direct current (DC).
The introduction of a third electrode, a grid between the filament and the plate, yields another function. A voltage applied to the grid controls the current flowing from the filament to the plate. Thus, it allows the device to be used as an electronic amplifier. Vacuum tubes are thus used for rectification, amplification, switching, or similar processing or creation of electrical signals.
The vast majority of modern day tubes consist of a sealed container with a vacuum inside, and essentially rely on thermionic emission of electrons from a hot filament or a cathode heated by the filament. Some exceptions to this are dealt with in the section about gas-filled tubes below.
Vacuum tubes were critical to the development of electronic technology, which drove the expansion and commercialisation of radio broadcasting, television, radar, sound reinforcement, sound recording and reproduction, large telephone networks, analogue 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 vacuum tube with three electrodes (called a triode) 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.
Source - Wikipedia