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 emissionelectrons 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, which caused the electron beams to act like a virtual suppressor grid to deflect some secondary electrons back to the anode. The beam tetrode 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, M-OV marketed this tube family as the "KT" series, standing for "kinkless tetrode". A number of different KT tubes were later marketed by M-OV. Some, but not all, were versions of existing American beam tetrode tubes or European power pentodes, such as the KT66, KT77, KT88, and KT63. Although the RCA6L6 of 1936 was the first true "beam power tube" on the market, the later KT66 became almost equally famous, at least in Europe. The two tubes were nearly interchangeable, except that the KT66 was somewhat more rugged than the early metal 6L6. The KT66 was very popular in European radios and audio amplifiers. It was the standard output tube in the classic Quad II and in the LEAKType 15 and TL/12, 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. However, the plate dissipation of the 6L6GC, at 30W, exceeds the KT66's 25W, and adjustment of the amplifier's bias is necessary. M-OV ceased glass vacuum tube manufacturing in 1988; their old audio tube types became valuable collectibles. In 2004 original M-OV KT66 tubes, unused in original carton, sold for US$250. KT66 tubes continued to be manufactured at Reflektor Saratov in Russia, JJ Electronic in Slovakia, and at Liuzhou in China. Some modern Russian manufacture Sovtek KT66 tubes are actually 6L6GC tubes in a KT66 style bottle. While these tubes have the same pinout and minimum tolerances required of a KT66 tube, they do not have the performance characteristics of a true kinkless tetrode KT66 tube. By contrast the very latest Russian manufactured tubes not only carry the same internal electrode structure as the original KT66 they also have the same rugged electrical characteristics and can withstand a high voltage on grid 2 comparable to the anode voltage rating, allowing greater power output afforded by higher voltage capability when run inultralinear connection.
