A big reason why the 7027 lasted longer is because it did have a harder vacuum than the 6L6GC's, sorry I forgot to mention that. In fact, thats a big reason why the EL34 doesn't last as long as your typical 6L6GC, despite the much superior votlage ratings, because of the much lower vacuum.
I've personally taken apart a dead RCA black plate 6L6GC and a dead RCA black plate 7027A, and they were identical inside. Same size plate, same stampings, same cathode, same grid locations, same getter, etc. So all I can conclude is that they were the same exact tube, with a different pin configuration on the 7027, and a harder vacuum on the 7027, which helped it to last longer.
Most Ampegs are ready to put a 6L6GC in without any changes except for bias. But it is a good idea to check anyway. The EL34 and 6550 both have higher filament current, so they work the power transformer harder, but thankfully Ampeg used generously sized power transformers usually. The other problem with using the 6550 is that it's max screen grid voltage is only 400v, and many Ampegs, and Sunns for that matter, have a higher voltage than that on the screens. GE's 6550A was rated for 660v plate and 440v screen, so it still isn't rated high enough for most Ampegs or Sunns on the screens. Now we know that many people have used them throughout the years and they hold up ok, but a KT 88 exceeds all the voltage specs, so in that respect its always a better choice for reliability.
Greg
