6CA7's ARE for all intents and purposes, EL34's electrically, though true 6CA7's are better tubes for most bass amps. Matter of fact, the Philips (many of which are actually Mullards) "6CA7's" that shipped with Traynors aren't even true 6CA7's; they're all just relabelled EL34's. The difference between an EL34 and 6CA7's is that a 6CA7 is a beam tetrode with a higher peak current capability, while the EL34 is a true pentode with an extra emission control grid.
Early YBA-3's (And YBA-1's and host of other Traynors) were initally equipped with 7027's, which are just 6L6GC's with an extra grid connection on the base; those require much less bias voltage and don't put out as much power and the two types CANNNOT be swapped without bias circuit modification.