Sylvania introduced the 6CA7 in the late fifties. It is very similar to the European EL34 and they are interchangeble in terms of functioning as power tubes! However Sylvania relied heavily on their 6L6GC designs when developing the 6CA7 so originally they were not the same. Some think of it as a cross between a 6L6GC and a EL34. 6CA7's were mostly used in hifi applications. Magnatone might have been the only guitar amp manufacturer to use them in the sixties.
So, comparing the sound of EL34s vs. 6CA7s in a guitar amp? let your ears be the judge.
Both JJ and Electro-Harmonix make a new 6CA7.