The 6L6 was first introduced by RCA in 1935 and interestingly enough, the first version had a metal envelope and was the first true beam tetrode ever made – ANYWHERE!
All the successive versions however, had a glass envelope and over the years we worked through the 6L6G, 6L6GA, 6L6GB, 5881, 5932, 7027, and the final version 6L6GC The voltage and power rating of the 6L6 series were gradually enhanced by the addition of features such as a Micanol base, thicker plates, thicker grid wires, grid cooling fins, and ultra-black anode coatings such that the original having a 19 watts dissipation was ‘hot-rodded’ to give 30W by the time 6L6GC came around.
Lateral development of the 6L6 occurred with the 6V6 being developed for domestic radio and car radio applications and of course the 807 for radio transmitter PA usage
6L6's working in class AB2 push pull had the potential to deliver 50 Watts for less than 2%THD – quite a feat!
This impressive performance allied to longevity in service makes them keenly sought for audio and guitar amplifier applications today.
Mullard entered the fray in 1945 with the introduction of the EL35, a Pentode equivalent to the 6L6, (itself overtaken and replaced by the EL37), a rare valve to be found today.