6L6 is the designator for a vacuum tube introduced by Radio Corporation of America in July 1936. At the time Philips had already developed and patented power pentode designs, which were fast replacing power triodes due to their greater efficiency.
History
The 6L6 is a 
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, MOV marketed this tube family under the sobriquet "KT", meaning "kinkless tetrode".
Because MOV's engineers did not feel the kinkless tetrode could be successfully mass
-produced, they licensed the design to RCA. This proved to be a poor business decision on MOV's part. RCA subsequently had enormous success with the 6L6. It replaced the use of power triodes in public-address amplifiers almost overnight. So many applications were found for the 6L6 that a complete list would be impossible to assemble. MOV introduced their version, the KT66, a year later.
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, MOV marketed this tube family under the sobriquet "KT", meaning "kinkless tetrode".
Because MOV's engineers did not feel the kinkless tetrode could be successfully mass
-produced, they licensed the design to RCA. This proved to be a poor business decision on MOV's part. RCA subsequently had enormous success with the 6L6. It replaced the use of power triodes in public-address amplifiers almost overnight. So many applications were found for the 6L6 that a complete list would be impossible to assemble. MOV introduced their version, the KT66, a year later.RCA's first version had a metal-canister shell rather than glass — being one of the early octal base tubes, most of which were marketed as having metal shells. Later versions, including the 6L6G, 6L6GA, 6L6GB, 5881, 5932, 7027, and the final version 6L6GC had glass envelopes, which made radiation cooling of the anode easier. The voltage and power rating of the 6L6 series was gradually pushed upwards by adding features such as a micanol base, thicker plates, thicker grid wires, grid cooling fins, and special ultra-black plate coatings. The original metal version was rated for 19 watts dissipation, while the later 6L6GC is usually rated for 30 watts.
Variations
Early variations included transmitting tubes such as the 807 (1937), the smaller 6V6 (1937), the many KT versions marketed in Europe, and a subsequent vast array of audio and RF power tubes. One of the largest post-WWII applications was in the basic design of television sweep power tubes, starting with the 6BG6 (1946), a modified 807. TV sweep tubes were not replaced by transistors in earnest, until the 1970s.
Further testimony for this device's success would be even simpler: the 6L6GC version is still being manufactured and is used, primarily, in guitar amplifiers. Manufacture continues in Russia (2 factories), China (2 factories), Slovakia and Serbia. Thus, the 6L6 has enjoyed one of the longest active lifetimes of any electronic component at more than 70 years.
Further testimony for this device's success would be even simpler: the 6L6GC version is still being manufactured and is used, primarily, in guitar amplifiers. Manufacture continues in Russia (2 factories), China (2 factories), Slovakia and Serbia. Thus, the 6L6 has enjoyed one of the longest active lifetimes of any electronic component at more than 70 years.
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