KT66
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KT66 is the designator for a vacuum tube introduced by Marconi-Osram Valve Co. Ltd. (M-OV) of Britain in 1937.
The KT66 is the direct descendant of the "Harries Valve" developed by British engineer J. Owen Harries and marketed by the Hivac Co. Ltd. in 1935. Harries is believed to be the first engineer to discover the "critical distance" effect, which maximized the efficiency of a power tetrode by positioning its anode at a distance which is a specific multiple of the screen grid-cathode distance. This design also minimized interference of secondary emission electrons dislodged from the anode.
EMI engineers Cabot Bull and Sidney Rodda improved the Harries design with a pair of beam plates, connected to the cathode, which directed the electron streams into two narrow areas and also acted like a suppressor grid to absorb some secondary electrons. The beam design was also undertaken to avoid the patents which the giant Philips firm held on power pentodes in Europe. Because this overall design eliminated the "tetrode kink" in the lower parts of the tetrode's voltage-current characteristic curves (which sometimes caused tetrode amplifiers to become unstable), M-OV marketed this tube family under the sobriquet "KT", meaning "kinkless tetrode".
A number of different KT tubes were marketed by M-OV thereafter. Some were versions of existing American beam tubes or European power pentodes, such as the KT66 (6L6), KT77 (EL34) and KT63 (6V6); while others were unique to the KT series.
Although the RCA 6L6 of 1936, the result of a license agreement between RCA and M-OV, was the first true "beam power tube" on the market, the later KT66 became almost equally famous. The two tubes were nearly interchangeable, except that the KT66 was much more rugged and capable of tolerating overvoltages and overcurrents that would destroy a 6L6.
The KT66 was very popular in European radios and audio amplifiers. It was the standard output tube in the classic Quad II (1952, a version of which is still being manufactured today) and in the LEAK Type 15 (1945) and TL/12 (1948), both among the earliest British hi-fi amplifiers. Because of their excellent electrical characteristics and overload tolerance, KT66s are preferred by some guitar players for use in guitar amps in place of 6L6GC. (Readjustment of the amplifier's bias is necessary.)
M-OV ceased glass vacuum tube manufacture in 1988, yet their old audio tube types have become valuable collectibles. As of 2004, an old M-OV KT66 (bearing the official "Genalex" marketing brand that M-OV used outside the UK), unused in its original carton, can easily bring US$250. The trade in KT66s is so intense that at least two copies are being manufactured; one at Reflektor Saratov in Russia, and one at Liuzhou in China. These modern tubes are often packaged in counterfeit "Genalex" boxes and sold as being the real thing.
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[edit] References
- Stokes, John, 70 YEARS OF RADIO TUBES AND VALVES, Vestal Press, NY, 1982, pp. .
- Thrower, Keith, HISTORY OF THE BRITISH RADIO VALVE TO 1940, MMA International, 1982, p. 59.
- "History of the 6L6", Eric Barbour, VACUUM TUBE VALLEY, issue 4 (1996), p. 3.
- Schade, O. H. "Beam Power Tubes", Proc. of IRE, Feb. 1938.
