The 6L6 family is a very old tube type. I believe audio was preety young, when this tube was developed in the USA. People in the audio industry wanted something more powerful than a triode, 2A3's or 300A or 300B. So this "Beam Power Tetrode was developed, it was known as 807, and had the Plate connection on top because of the high voltage requirement. From this design the 6L6 evolved. Several variants soon were developed , 6L6G and then 6L6GB and finally the 6L6GC, also the 5881 cam into the picture.> It was so successful that most amplifiers made in the USA used this tube to great extent from 1930 to the late 1970. Users of 6l6 where McIntosh, Scott, Fisher, Heath, Eico etc. and many many guitar amplifiers used them too like Fender, Ampeg.
Meanwhile in Europe, anothe tube was making the raves, that was the EL34. The EL34 IS A PENTODE, SLIGHTLY DIFFENT FROM 6L6 which is A BEAM TETRODE, but pratically just as efficient. But as competition goes it was a one upmanship game, the EL34 had a higher plate voltage (800V) versus the 6L6GC and had a higher power rating. It was so popular in Europe that the Americans made its own version too, the 6CA7. This tube was popularized in the US by Dynaco in its famous ST70s.
They are different tubes therefore they sound different too. They have the same PIN connections but differnt ratings and bias requirements, but they are are also similar in some aspects.
