EL 34 vs. KT 88 ---- What's the difference?

Why can some amps use EL34 or KT88 while other amps cannot use them interchangeably?

Does the output transformer have to "match" the tubes? If yes, what are the impedance differences?

Some amps can use both el34s and 88s b/c they have variable bias, allowing you to adjust the bias to the appropriate level for different tubes.

Stewie, I think it is not that simple. Different plate voltage and the bias range of the fixed bias circuit both come in to play I think. Just having the ability to adjust bias is not enuf. A user should always ask the manufacturer if use is possible, or at a minimum a knowledgable techie (which I am not) who knows the amp before trying different tube types.

Is there anyone who has an amp that is made to handle both EL34 and KT88 and prefers the EL34 generally, that is if you had to make a choice you would stick with EL34 over using the KT88. I think one of my amps (Music Reference RM9 SE) was built around the Siemens EL34 tube, but I still think it sounds better with the Genelex KT88 reissues.

Pubul57--My Dynaco mk iiis take both el34s and kt88s, but they're powering only the midrange/tweeter cabinets of my VR4 HSEs. Also, my kt88s are EH and the 34s are the winged c variety. That said, despite the fact that the fat bottle kt88s are the most pleasing to look at (not that anyone cares about that!), I find myself going back to the el34s. The difference, to my ears, is slight, and I doubt most could hear the difference if they weren't trying to hear it. But there is something special about the el34 midrange, at least in my system--not just more of it, but a different sonic character. Liquid is a word often used, and that gets at it.