Vacuum tube

In electronics the vacuum tube or thermionic) valve (outside the USA) is a device generally used to amplify, modify or control a signal. Mostly the vacuum tube has been replaced by much smaller, more reliable and less expensive solid state technology. Tubes are still used in several specialized applications such as audio systems and high power RF transmitters.

Simply put vacuum tubes are arrangements of electrodes in a vacuum within an insulating, temperature-resistant envelope, (most often of glass). Except for some special-purpose tubes which use a gas in the envelope. The electrodes are attached to leads which pass through the envelope via an air tight seal which are designed to plug into a tube socket for easy replacement. A filament (the cathode) is heated so that it releases electrons into the vacuum (thermionic emission). These are drawn to a metal plate (the anode) inside the envelope. Another electrode (the grid) is placed between the filament and plate. A voltage applied to the grid will control the plate plate current.