6L6 Family
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One of a wide range of octal-based beam power pentodes, the original 6L6 was a powerful audio output tube with the ability to dissipate around 20 watts. It could be mounted in any position but required substantial ventilation. Some of the more common equivalents and/or substitutions include the KT-66 (European version), the 1614 and 1622 (a premium industrial version of the 6L6 metal type), types 7581, 5881and EL37.

A descendant of the "Harries Valve" developed by British engineer J. Owen Harries and marketed by the Hivac Co. Ltd. in 1935, the 6L6 was introduced by the Radio Corporation of America (RCA) in July 1936. Philips had already developed and patented power pentode designs (most notably the EL34) and the more efficient power pentodes were quickly replacing power triodes. RCA wanted a slice of the action, but rather than copy Philip's design they purchased the rights for the "kinkless tetrode" or Beam tetrode from British firm Marconi & Osram Valve (MOV) through a design share agreement, although RCA's own engineers were developing similar designs at that time. Because MOV's engineers did not feel the kinkless tetrode could be successfully mass-produced, they were happy to license the design to RCA - a poor business decision on MOV's part. RCA subsequently had enormous success with the 6L6 which replaced the use of power triodes in public-address amplifiers almost overnight. MOV bounced back a year later when they introduced their own version, the KT66 which was used in the extremely popular QUAD II power amplifier.

L-R early metal-type; early 60's 6L6GC, current version

RCA's first 6L6 had a metal-canister shell rather than glass; most of the early octal-based tubes of the era were being manufactured with metal shells and many radio users were nervous about being injured by glass from a broken tube. Later versions, including the 6L6G, 6L6GA, 6L6GB, 5881, 5932, and the final version 6L6GC had glass envelopes, which facilitated radiation cooling of the anode. The original metal version was rated for 19 watts dissipation, while the 6L6GC was capable of 30 watts.

The list of variations of the 6L6 design is quite extensive. Early variations included transmitting tubes such as the 807 (1937) and the giant 813 (1938), the smaller 6V6 (1937) not to mention the many KT versions marketed in Europe. 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. Interestingly, transistors did not fully replace TV sweep tubes until the 1970s.

Classic amps using the 6L6 / 6L6GC
McIntosh MC-240
Fisher 80-AZ & 100-AZ
Sansui AU-111
Grommes Little Jewel LJ-6
Scott 210-C/D, 220-A & 223
Harman Kardon A-250 'Epic'

The success of the 6L6GC is clearly illustrated by the fact that after 70 years it is still being used (primarily in guitar amplifiers) and manufactured by numerous factories in Russia, China, Slovakia and Serbia.