Vacuum tube
A vacuum tube, electron tube,[1][2][3] valve (British usage), or tube (North America),[4] is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
This article is about the electronic device. For experiments in an evacuated pipe, see Free fall. For the transport system, see Pneumatic tube. For Blood sampling, see Vacutainer.
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.
The simplest vacuum tube, the diode (i.e. Fleming valve), was invented in 1904 by John Ambrose Fleming. It contains only a heated electron-emitting cathode and an anode. Electrons can flow in only one direction through the device—from the cathode to the anode. Adding one or more control grids within the tube allows the current between the cathode and anode to be controlled by the voltage on the grids.[5]
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, cooler, 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.
Thermionic tubes are still employed in some applications, such as the magnetron used in microwave ovens, certain high-frequency amplifiers, and high end audio amplifiers, which many audio enthusiasts prefer for their "warmer" tube sound, and amplifiers for electric musical instruments such as guitars (for desired effects, such as "overdriving" them to achieve a certain sound or tone).
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.
Other classifications are:
Vacuum tubes may have other components and functions than those described above, and are described elsewhere. These include as cathode-ray tubes, which create a beam of electrons for display purposes (such as the television picture tube, in electron microscopy, and in electron beam lithography); X-ray tubes; phototubes and photomultipliers (which rely on electron flow through a vacuum where electron emission from the cathode depends on energy from photons rather than thermionic emission).
Names[edit]
The generic name "[thermionic] valve" used in the UK derives from the unidirectional current flow allowed by the earliest device, the thermionic diode emitting electrons from a heated filament, by analogy with a non-return valve in a water pipe.[66] The US names "vacuum tube", "electron tube", and "thermionic tube" all simply describe a tubular envelope which has been evacuated ("vacuum"), has a heater and controls electron flow.
In many cases, manufacturers and the military gave tubes designations that said nothing about their purpose (e.g., 1614). In the early days some manufacturers used proprietary names which might convey some information, but only about their products; the KT66 and KT88 were "kinkless tetrodes". Later, consumer tubes were given names that conveyed some information, with the same name often used generically by several manufacturers. In the US, Radio Electronics Television Manufacturers' Association (RETMA) designations comprise a number, followed by one or two letters, and a number. The first number is the (rounded) heater voltage; the letters designate a particular tube but say nothing about its structure; and the final number is the total number of electrodes (without distinguishing between, say, a tube with many electrodes, or two sets of electrodes in a single envelope—a double triode, for example). For example, the 12AX7 is a double triode (two sets of three electrodes plus heater) with a 12.6V heater (which, as it happens, can also be connected to run from 6.3V). The "AX" designates this tube's characteristics. Similar, but not identical, tubes are the 12AD7, 12AE7...12AT7, 12AU7, 12AV7, 12AW7 (rare), 12AY7, and the 12AZ7.
A system widely used in Europe known as the Mullard–Philips tube designation, also extended to transistors, uses a letter, followed by one or more further letters, and a number. The type designator specifies the heater voltage or current (one letter), the functions of all sections of the tube (one letter per section), the socket type (first digit), and the particular tube (remaining digits). For example, the ECC83 (equivalent to the 12AX7) is a 6.3V (E) double triode (CC) with a miniature base (8). In this system special-quality tubes (e.g., for long-life computer use) are indicated by moving the number immediately after the first letter: the E83CC is a special-quality equivalent of the ECC83, the E55L a power pentode with no consumer equivalent.
Vacuum tubes in the 21st century[edit]
Industrial, commercial, and military niche applications[edit]
Although vacuum tubes have been largely replaced by solid-state devices in most amplifying, switching, and rectifying applications, there are certain exceptions. In addition to the special functions noted above, tubes still have some niche applications.
In general, vacuum tubes are much less susceptible than corresponding solid-state components to transient overvoltages, such as mains voltage surges or lightning, the electromagnetic pulse effect of nuclear explosions,[94] or geomagnetic storms produced by giant solar flares.[95] This property kept them in use for certain military applications long after more practical and less expensive solid-state technology was available for the same applications, as for example with the MiG-25.[94]
Vacuum tubes are practical alternatives to solid-state devices in generating high power at radio frequencies in applications such as industrial radio frequency heating, particle accelerators, and broadcast transmitters. This is particularly true at microwave frequencies where such devices as the klystron and traveling-wave tube provide amplification at power levels unattainable using current semiconductor devices. The household microwave oven uses a magnetron tube to efficiently generate hundreds of watts of microwave power. Solid-state devices such as gallium nitride are promising replacements, but are very expensive and in early stages of development.
In military applications, a high-power vacuum tube can generate a 10–100 megawatt signal that can burn out an unprotected receiver's frontend. Such devices are considered non-nuclear electromagnetic weapons; they were introduced in the late 1990s by both the U.S. and Russia.