Summary
The KT66 is a high slope, indirectly heated beam triode designed principally
for use in the output stage of audio amplifiers. It may also be used
as an oscillator or R.F. power amplifier for frequencies up to 30 Mc/s.
It is suitable for either single-ended or push-pull audio operation, and may be employed as a triode with screen connected through a 100-ohm resistor to the annode.
History
The KT66 was introduced by Marconi-Osram Valve
Co. Ltd. (M-OV) of Britain in 1937, a 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 1936 RCA 6L6 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 Leak 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 the real
thing.
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