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Vacuum tubes were widely used in electronic devices before transistors and integrated circuits. They were invented after Thomas Edison observed the Edison Effect. Vacuum tubes are still used in some specialized devices, including military applications and electric guitar amplifiers. They contain filaments that release electrons, creating a negatively charged electron cloud. Some tubes also have a small screen grid to amplify voltage. However, vacuum tubes have drawbacks, including instability over time and damage if air enters the tube.
Before being replaced by transistors and integrated circuits, vacuum tubes (thermionic valves) were predominantly used in electronic devices such as televisions, radios and computers. They are still in use today in some specialized devices.
The invention of vacuum tubes dates back to the observation of the so-called Edison Effect, an observation made by Thomas Edison. Edison noticed that current flows between the filament of an incandescent lamp and a plate in a vacuum when the plate is connected to the positive end of the filament.
Because early computers ran on vacuum tubes instead of today’s tiny computer chips, a single computer had to contain thousands of vacuum tubes and could fill an entire room. Early stereo amplifiers also used vacuum tubes, and some audiophiles still prefer them today because they produce less distortion. Vacuum tubes are still used in some electric guitar amplifiers. Additionally, the tubes are still used in some military applications, because the electronics of the tubes are unaffected by the radio waves produced by atomic explosions.
Vacuum tubes are usually encased in a glass envelope, although some tubes use ceramic or metal instead. In its most basic diode design, the tube, or envelope, is hermetically sealed to create a vacuum. The electrodes inside the pouch are attached to cables, which protrude from the pouch and plug into an outlet. A simple vacuum tube contains filaments inside its casing, similar to those in a light bulb. The filaments are heated and then release electrons, creating a negatively charged electron cloud. The electrons are attracted to an anode, or small metal plate, inside the tube, which is positively charged, and a unidirectional flow is established between the filament and the plate.
Sometimes an additional electrode in the form of a small screen grid is contained in the tube, which is then called a triode, which is more efficient and able to amplify the voltage. When voltage is applied to the grid, the flux between the filament and the plate can be varied. In addition to diodes and triodes, further innovations have followed, including tetrodes, hexodes, heptodes and octodes, designed for a variety of special applications and to minimize distortion. Some vacuum tubes combine the function of two or more diodes or triodes into a single unit.
One major drawback of vacuum tubes is that the filament becomes unstable over time. Also, if air escapes into the tube, the oxygen will react with the hot filament and damage it. A tube’s properties change with age, which is why early vacuum tube televisions had to be adjusted often to produce a good picture.
