Thermionic Valve

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The valve consists of an evacuated glass bulb containing two electrodes: a cathode in the form of a \"filament\", a loop of carbon or fine tungsten wire, similar to that used in the light bulbs of the time, and an anode (plate) consisting of a sheet metal plate. Although in early versions the anode was a flat metal plate placed next to the cathode, in later versions it became a metal cylinder surrounding the cathode. In some versions, a grounded copper screen surrounded the bulb to shield against the influence of external electric fields.

In operation, a separate current flows through the cathode \"filament\", heating it so that some of the electrons in the metal gain sufficient energy to escape their parent atoms into the vacuum of the tube, a process called thermionic emission. The AC to be rectified is applied between the filament and the plate. When the plate has a positive voltage with respect to the filament, the electrons are attracted to it and an electric current flows from filament to plate. In contrast, when the plate has a negative voltage with respect to the filament, the electrons are not attracted to it and no current flows through the tube (unlike the filament, the plate does not emit electrons). As current can pass through the valve in one direction only, it therefore \"rectifies\" an AC to a pulsing DC current.

This simple operation was somewhat complicated by the presence of residual air in the valve, as the vacuum pumps of Fleming's time were unable to create as high a vacuum as exists in modern vacuum tubes. At high voltages, the valve could become unstable and oscillate, but this occurred at voltages far above those normally used.

The Fleming valve was the first practical application of thermionic emission, discovered in 1873 by Frederick Guthrie. As a result of his work on the incandescent lamp in 1880, Thomas Edison discovered that heated material from the negative electrode (later discovered to be electrons) was moving through the vacuum and collecting on the positive electrode, which led to it being called the Edison effect. Edison was granted a patent for this device as part of an electrical indicator in 1884, but did not find a practical use for it. Professor Fleming of University College London consulted for the Edison Electric Light Company from 1881-1891, and subsequently for the Marconi Wireless Telegraph Company.

The receiver for the transatlantic demonstration employed a coherer, which had poor sensitivity and degraded the tuning of the receiver. This led Fleming to look for a detector which was more sensitive and reliable while at the same time being better suited for use with tuned circuits.[2][3] In 1904 Fleming tried an Edison effect bulb for this purpose, and found that it worked well to rectify high frequency oscillations and thus allow detection of the rectified signals by a galvanometer. On November 16, 1904, he applied for a US patent for what he termed an oscillation valve. This patent was subsequently issued as number 803,684 and found immediate utility in the detection of messages sent by Morse code. The Fleming valve was used by the Marconi company in its shipboard receivers until around 1916, when it was replaced by the triode.

The Fleming valve proved to be the start of a technological revolution. After reading Fleming's 1905 paper on his oscillation valve, American engineer Lee de Forest in 1906 created a three-element vacuum tube, the Audion, by adding a wire grid between cathode and anode. It was the first electronic amplifying device, allowing the creation of amplifiers and continuous wave oscillators. De Forest quickly refined his device into the triode, which became the basis of long-distance telephone and radio communications, radars, and early digital computers for 50 years, until the advent of the transistor in the 1960s. Fleming sued De Forest for infringing his valve patents, resulting in decades of expensive and disruptive litigation, which were not settled until 1943 when the United States Supreme Court ruled Fleming's patent invalid.[4]

Later, when vacuum tube equipment began to be powered from AC electrical outlets instead of DC batteries, the Fleming valve was developed into a rectifier to produce the DC plate (anode) voltage required by other vacuum tubes. Around 1914 Irving Langmuir at General Electric developed a high voltage version called the Kenotron which was used to power x-ray tubes. As a rectifier, the tube was used for high voltage applications but its low perveance made it inefficient in low voltage, high current applications. Until vacuum tube equipment was replaced by transistors in the 1970s, radios and televisions usually had one or more diode tubes.

Vacuum Tube / Thermionic Valves Includes: Basics How does a tube work Vacuum tube electrodes Diode valve / tube Triode Tetrode Beam Tetrode Pentode Equivalents Pin connections Numbering systems Valve sockets / bases Travelling wave tube Vacuum tube or thermionic valve technology has been in use since just after the beginning of the twentieth century.

The first observations of what eventually developed into vacuum tube or thermionic valve technology were observed by Edison. In his quest to manufacture better incandescent light bulbs, he noticed and effect that was later called the Edison Effect.

The next major development occurred when Lee de Forest added a third electrode called a grid. This opened up the basic thermionic valve or vacuum tube idea to amplify signals and provide considerably more functionality.

The first vacuum tube / thermionic valve was developed when Ambrose Fleming used a discovery Edison had made that was called the Edison Effect. Edison had not been able to find any applications for it, but Fleming used this two electrode diode it to rectify radio signals. Later Lee de Forest added a third electrode to make a triode. Further developments improved performance and added additional electrodes.

The concept of thermionic valve or vacuum tubes used the idea that a heated element in a vacuum emitted electrons that would normally remain in the vicinity of this heated element because of the charge attraction.

There are several different types of diode valve. Each has its own properties and can be sued for different applications. Diodes can be used for rectifying, whilst triodes, tetrodes and pentodes are normally used in amplification applications. Heptodes are often used in RF mixers. As a result it is necessary to look at the different types when any application is needed.

The various different types of vacuum tube / thermionic valve enable a variety of different functions to be provided by these devices. Diodes are obviously very different to triodes, etc, but the other types of valve can be used in different circuits as required by the different applications.

The type known as a thermionic tube or thermionic valve utilizes thermionic emission of electrons from a hot cathode for fundamental electronic functions such as signal amplification and current rectification. Non-thermionic types such as a vacuum phototube, however, achieve electron emission through the photoelectric effect, and are used for such purposes as the detection of light intensities. In both types, the electrons are accelerated from the cathode to the anode by the electric field in the tube.

These devices became a key component of electronic circuits for the first half of the twentieth century. They were crucial to the development of radio, television, radar, sound recording and reproduction, long-distance telephone networks, and analog and early digital computers. Although some applications had used earlier technologies such as the spark gap transmitter for radio or mechanical computers for computing, it was the invention of the thermionic vacuum tube that made these technologies widespread and practical, and created the discipline of electronics.[6]

In the 1940s, the invention of semiconductor devices made it possible to produce solid-state devices, which are smaller, more efficient, reliable, durable, safer, and more economical than thermionic tubes. Beginning in the mid-1960s, thermionic tubes were being replaced by the transistor. However, the cathode-ray tube (CRT) remained the basis for television monitors and oscilloscopes until the early 21st century.

Not all electronic circuit valves / electron tubes are vacuum tubes. Gas-filled tubes are similar devices, but containing a gas, typically at low pressure, which exploit phenomena related to electric discharge in gases, usually without a heater.

One classification of thermionic vacuum tubes is by the number of active electrodes. A device with two active elements is a diode, usually used for rectification. Devices with three elements are triodes used for amplification and switching. Additional electrodes create tetrodes, pentodes, and so forth, which have multiple additional functions made possible by the additional controllable electrodes.

Tubes have different functions, such as cathode-ray tubes which create a beam of electrons for display purposes (such as the television picture tube) in addition to more specialized functions such as electron microscopy and electron beam lithography. X-ray tubes are also vacuum tubes. Phototubes and photomultipliers rely on electron flow through a vacuum, though in those cases electron emission from the cathode depends on energy from photons rather than thermionic emission. Since these sorts of \"vacuum tubes\" have functions other than electronic amplification and rectification they are described elsewhere.

The earliest vacuum tubes evolved from incandescent light bulbs, containing a filament sealed in an evacuated glass envelope. When hot, the filament releases electrons into the vacuum, a process called thermionic emission, originally known as the Edison effect. A second electrode, the anode or plate, will attract those electrons if it is at a more positive voltage. The result is a net flow of electrons from t