I'm pretty familiar with the "big four" American and British style power tubes: 6L6, 6V6, EL34 and EL84. On and off over the years, I've seen mention of the KT66 tube. Is this just a Euro equivalent to the 6L6, or is it a different tube entirely? As I understand it, there are Euro and American designations for what amounts to the same tube (e.g. 12AX7 and ECC83). Is this the case with the KT66 and 6L6, or are they actually different? If they are different, can anybody explain how they're different, particularly in terms of guitar amp tone? Also, how the heck does teh 6550 fit into this mess?