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vacuum tube
Electron tube consisting of a sealed glass or metal enclosure from which the air has been withdrawn. It was used in early electronic circuitry to control a flow of electrons. In the first half of the 20th century, vacuum tubes allowed the development of radio broadcasting, long-distance telephone service, television, and the first electronic digital computers, which were the largest vacuum-tube systems ever built. Transistors have replaced them in virtually all applications, but they are still occasionally used in display devices for television sets and computers (cathode-ray tubes), in microwave ovens, and as high-frequency transmitters on space satellites.
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| Early Vacuum Tube |
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| Early vacuum tubes were used to amplify signals for radio and other audio devices. This one was made in 1915. Tubes were not used as switches in calculating machines until 1939. (Image courtesy of AT&T.) |
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| Tubes in the 21st Century |
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| Vacuum tubes are still used and not just for CRTs. Many audiophiles swear that tubes are the only electronics that can faithfully reproduce musical sounds, and they would be delighted with these Model One amplifiers from Red Rose Music (www.redrosemusic.com). Designed by legendary audio engineer Mark Levinson, for a mere $25,000, you can own all 212 pounds of this top-of-the-line equipment. |
vacuum tube [′vak·yəm ‚tüb]
| vacuum tube - electron tube |
Vacuum Tube
(in Russian, elektronnaia lampa), an electron-tube device whose operation is based on a change in a stream of electrons emitted from a cathode and traveling in a vacuum by an electric field formed by means of electrodes. Depending on their output power, vacuum tubes are classified as receiving tubes or as oscillator tubes. The output power of receiving tubes is not higher than 10 watts (W); that of oscillator tubes is higher than 10 W.
The first vacuum tubes—vacuum diodes and triodes—were developed in the early 20th century on the basis of the production technology for incandescent lamps. They very much resembled incandescent lamps in appearance, having a glass bulb with a tungsten filament serving as the cathode in the center. (The word lampa [“lamp”] in the Russian term elektronnaia lampa underscores the similarity between early vacuum tubes and incandescent lamps; the adjective elektronnaia [“electron”] indicates the fundamental differences between the two. Although the appearance of vacuum tubes had changed considerably as early as the 1930’s, so that vacuum tubes no longer resembled incandescent lamps, the word lampa has been retained to this day in the Russian term.)
In the first half of the 20th century, vacuum tubes had a decisive influence on the evolution of radio engineering. Such tubes served as the basis for the development of radio communication, radio broadcasting, television, radar, and—for first-generation computers—computer technology. Between 1921 and 1941, the annual world output of vacuum tubes increased from a million to hundreds of millions. However, advances in semiconductor electronics made the further development of radio equipment based on receiving tubes no longer worthwhile. In the 1960’s and 1970’s, the development of such equipment was discontinued. As a result, the annual world production of receiving tubes decreased by roughly a factor of three between 1960 and 1975.
Nevertheless, advances in semiconductor electronics did not affect the development of oscillator tubes, since the output power of radio-frequency semiconductor devices does not exceed 10–100 watts. The oscillator tubes produced today include triodes and tetrodes. They are characterized by a power rating of 50 W to 3 megawatts (MW) in the continuous mode and of up to 10 MW in the pulsed mode. In the development of new types of oscillator tubes, primary attention is given to such considerations as reducing the grid current and raising the power amplification factor to 25–30 decibels (dB). Another consideration is the linearity of the grid-plate transfer characteristic, that is, the linearity of the dependence of the plate current on the control-grid voltage; in present-day tubes, third-order distortions have been reduced to –45 dB. A fourth consideration is increasing the efficiency of oscillator tubes. For example, in triodes with magnetic electron focusing, the efficiency may be as high as 90 percent. Such triodes are used for high-frequency heating.
REFERENCES
Vlasov, V. F. Elektronnye i ionnye pribory, 3rd ed. Moscow, 1960.Yingst, T., et al. “Lampy bol’shoi moshchnosti s setochnym upravleniem—1972 g.” Trudy Instituta inzhenerov po elektrotekhnike i radioelektronike, 1973, vol. 61, no. 3, pp. 121–52. (Translated from English.)
Kleiner, E. Iu. Osnovy teorii elektronnykh lamp. Moscow, 1974.
V. F. KOVALENKO
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