I don`t have experience with KT77 (too rare/expensive), but I have some with various EL34. It is actually "kinkless tetrode", or beam power tube - EL34 is "true" pentode. (Is this a problem in practice - not really.) Almost every tube manual lists KT77 as EL34 "drop in" substitute, and many people observe it like "premium" version/replacement. You must check (and eventually fix) the bias when you install the new tubes, anyway. Hey, if you have some source with many cheap NOS KT77 - I`m in!