The 6L6GC is the typical power amp tube of US guitar amplifier brands. Originally produced by RCA since 1936 as 6L6 tube (24 watts) as a beam-power tetrode with steel envelope, it was further developed into the 6L6G, 6L6GA, 6L6GB and finally the 6L6GC (30 watts), all in the glass version still in use today. Series with smaller glass bulb are the 5881 / 6L6WGB (23 Watt) and the 6L6WGC (30 Watt). The European parallel type KT66 was already used in Marshall's JTM45 guitar amplifiers of the 1960s and in hi-fi audio amplifiers such as the Quad II.
Since the late 1940s, guitar amplifiers from Fender Musical Instruments used the 6L6G in the popular Super Amp® models and the 5881 in the Bassman® amp. From 1960 onwards, the more powerful 6L6GC was increasingly used in new models and achieved worldwide fame with its use in classic amplifiers such as Fender Twin Reverb®, Fender Super Reverb® and Fender Bassman®, as well as in the Mesa® Boogie Mark I. Today, the 6L6GC is probably the most widely used output tube in guitar amplifiers ever.