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In electronics, the vacuum tube can perfrom a function similar to the transistor. Vacuum tubes are now used only in specialized applications. For most purposes, integrated circuits and discrete semiconductor devices are less expensive, longer-lasting and more rugged.

Vacuum tubes, or tubes or thermionic valves are arrangements of electrodes surrounded by vacuum within an envelope. Although the envelope was classically glass, power tubes often use ceramics, and military tubes often use metal.

Tubes resemble incandescent light bulbs. Tubes have a filament heated by an electric current. When hot, the filament releases electrons into the vacuum. These electrons are electrostatically drawn to a positively charged outer metal plate called the anode, or more commonly just the plate. Electrons do not flow from the plate back toward the filament, even if the charge on the plate is made negative, because the plate is not heated. The arrangement of a filament and plate is called a diode and invented in 1904 by John Ambrose Fleming scientific adviser to the Marconi company based on an observation by Thomas Edison.

The next innovation, due to Lee DeForest in 1907, was to place another electrode, the grid, between the filament and plate. The grid is a bent wire or screen. De Forest discovered that the current flow from filament to plate depended on the voltage of the grid, and that the current drawn by the grid was very low. The resulting three-electrode device, the triode, was therefore an excellent amplifying device.

Many further innovations followed. It became common to use the filament to heat a separate electrode called the cathode, and to use the cathode as the source of electron flow in the tube rather than the filament itself. It was discovered that additional grids could be used to lower the current draw of the triode's control grid, and to improve the linear response of the tube. The triode gave way to the tetrode and later the pentode.

Tubes with 4, 5, 6, or 7 grids, called hexodes, heptodes, octodes, and nonodes, were generally used for frequency conversion in superheterodyne receivers. The heptode, or pentagrid, was the most common of these. 6BE6 is an example of a heptode.

It was common practice in some tube types to include more than one group of elements in one bulb. For instance, type 6SN7 is a "dual triode" which, for most purposes, can perform the functions of two triode tubes, while taking up half as much space and costing less.

The beam power tube is usually a tetrode with the addition of "beam forming plates". These plates direct the electron stream to certain spots on the anode, and thus overcome some design barriers to designing high power, high perfromance power tubes. 6L6 is a beam power tube.

The chief reliability problem of a tube is that the filament burns out. To increase life, tube designers try to run filaments at lower temperatures. To encourage electron emission at lower temperatures, filaments are coated, usually with thorium.

Another important reliability problem is that the tube fails when air leaks into the tube. Usually the air reacts chemically with the hot filament. Designers therefore worked hard to develop tube designs that sealed reliably. This was much of the reason why many tubes were constructed of glass. Metal alloys and glasses had been developed for light bulbs that expanded and contracted the same amounts when hot. These made it easy to construct an insulating envelope of glass, and pass wires through the glass to the electrodes and filament.

Tubes usually have glass envelopes, but metal and ceramic are possible choices. The nuvistor is a tiny tube made only of metal and ceramic. In some tubes, the metal envelope is also the anode. 4CX800 is an external anode tube of this sort.

Near the end of World war II, to make radios more rugged, some aircraft and army radios began to integrate the tube envelopes into the radio's molded aluminum or zinc chassis. The radio became just a printed circuit with non-tube components, that was soldered to the chassis that contained all the tubes.

Tubes were ubiquitous in the early generations of electronic devices, such as radios, televisions, and early computers. They are still used for specialised audio amplifiers, notably for electric guitar amplification, and for very high-powered applications such as microwave ovens and signal amplification for broadcast radio.

Other vacuum tube electronic devices include the magnetron, klystron and cathode ray tube. The magnetron is the most common type of tube in microwave ovens.

Other tube devices

Specialist low-pressure gas-filled tube devices include the Nixie tube and the dekatron. These are used to display numerals.

One of the proposed designs for a fusion reactor is basically a tube, the Farnsworth-Hirsch Fusor.

See also: Irving Langmuir

References:

• http://www.marconicalling.com/museum/html/events/events-i=39-s=0.html
• Plenty of interesting information about vacuum tubes at http://www.svetlana.com/docs/tubeworks.html.
• A lot of very interresting technical informations about vacuum tubes. Pdf files from old books in both english and german with an outstanding theory at http://www.radau5.ch/valves.html The difference between the american and the german technique is very interresting. The american technique use usually the gain as central parameter in the calculation. The german technique use the transparency (durchgriff) as the central parameter, that is a little bit more abstract but the transparency is the most constant of all the parameter of the tube, the result is at it is easyer to do predictible and precise calculations with it.