8974 / X-2159 is a power

tetrode A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). Th ...

designed for megawatt power levels in industrial and broadcast applications.

Specifications

The 8974 is an external anode tube made of metal and ceramic. It has an overall height of , a diameter of , and weighs . It contains two directly heated thoriated-tungsten filaments rated at 16.3 volts at approx. 1300 amperes for both filaments. The anode (plate) is designed to dissipate 1.5 MW. The tube may be operated as a

class C amplifier In electronics, power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier's characteristics and performance. The classes are related to the time period that the active ...

in CW mode, where a single tube with an anode voltage of 22.5 kV DC can provide up to 2,158 kW of RF power.data sheet
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Internal construction

The thoriated-tungsten filament consists of two independent sections mounted on water-cooled supports. The two filaments may be excited in quadrature to reduce hum contributed by an AC power source. Each filament is rated at a nominal value of 18.5 V at approx. 650 A for a total of over 20 kW of filament power required when the tube is in operation.

History

Experimental type X2159 was assigned to the development of a power tetrode by

Eimac Eimac is a trade mark of Eimac Products, part of the Microwave Power Products Division of Communications & Power Industries. It produces power vacuum tubes for radio frequency applications such as broadcast and radar transmitters. The company name i ...

(then a division of Varian Associates) on May 28, 1970, and the design engineer was Sterling G. McNees. This electron tube was intended for use in very high power medium-frequency broadcast service and VLF communications equipment and as a pulse modulator (as a switch tube). The EIA (

Electronic Industries Alliance The Electronic Industries Alliance (EIA; until 1997 Electronic Industries Association) was an American standards and trade organization composed as an alliance of trade associations for electronics manufacturers in the United States. They devel ...

) designation 8974 was assigned to this as it became a standard product. The initial technical data sheet was printed in July 1971.

Continental Electronics Continental Electronics is an American manufacturer of broadcast and military radio transmitters, based in Dallas, Texas. Although Continental today is best known for its FM, shortwave, and military VLF transmitters, Continental is most signific ...

of Dallas, Texas, designed two 8974s (one as PA and the other as modulator) into their D323 series 1 megawatt AM transmitters which were sold primarily into the middle East, where stations were built to cover large geographical areas. The 8974 is still used in the same transmitters , over 30 years later. The 8974 was also used as the basis for the development of several large tetrode switch tubes capable of operating at anode voltage up to 175 kV DC.

Cooling

The anode is cooled by circulated water via two

flange A flange is a protruded ridge, lip or rim (wheel), rim, either external or internal, that serves to increase shear strength, strength (as the flange of an iron beam (structure), beam such as an I-beam or a T-beam); for easy attachment/transfer of ...

s located at the top of the tube. The heat is transferred to the outside environment using a

radiator Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics. A radiator is always a ...

, or to a secondary cooling system using a

heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...

. Controlling the purity of the water is important to prevent the formation of copper oxide which would reduce the cooling efficiency. Impurities could also cause

electrolysis In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from n ...

which could destroy the cooling passages.EIMAC Care and Feeding of Power Grid Tubes, 5th edition, 2003, p. 149-150 The plumbing connections for the water inlet and outlet are made at the anode, where a high DC voltage (up to 22 kV DC) is present. Forced air is used to provide additional cooling to the filament and grid connections at the bottom of the tube.

References

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External links

* http://www.cpii.com/docs/datasheets/78/8974.pdf * http://www.hawkins.pair.com/nss.shtml While not an exact match to the 8974, this page contains a pictures of similar tube installed in a high power radio transmitter. * https://web.archive.org/web/20130627034533/http://home.comcast.net/~n6jv/8974.html Vacuum tubes