What is a Vacuum Tube ?
In electronics, a vacuum tube (U.S. and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. For most purposes, the vacuum tube has been replaced by the much smaller and less expensive transistor, either as a discrete device or in an integrated circuit. However, tubes are still used in several specialized applications such as guitar amplifiers (also called a valve amp outside the U.S.) and high power RF transmitters, as a display device in television sets and in microwave ovens.
Vacuum tubes, or thermionic valves, are arrangements of electrodes in a vacuum within an insulating, temperature-resistant envelope. Although the envelope was classically glass, power tubes often use ceramic and metal. The electrodes are attached to leads which pass through the envelope via an air tight seal. On most tubes, the leads are designed to plug into a tube socket for easy replacement.
The simplest vacuum tubes resemble incandescent light bulbs in that they have a filament sealed in a glass envelope which has been evacuated of all air. When hot, the filament releases electrons into the vacuum: a process called thermionic emission. The resulting negatively-charged cloud of electrons is called a space charge. These electrons will be drawn to a metal "plate" inside the envelope if the plate (also called the anode) is positively charged relative to the filament (or cathode). The result is a current of electrons flowing from filament to plate. This cannot work in the reverse direction because the plate is not heated and cannot emit electrons. In it's simplest form a vacuum tube can be created to operate as a diode: a device that conducts current only in one direction. A third element called a "control grid" can be added to the design which provides the ability to amplify a signal. Other configurations are also possible including the Pentode, a tube with 5 active elements providing an additional amplification factor. There are a large number of tube varieties and uses. This is only a very brief overview and we suggest consulting additional resources if you are interested in additional information. Some Information provided by wikipedia.org.