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

Also the Tung-Sol 12AX7s are in my opinion, the best of the affordable 12AX7s for all Jolida equipment. I am also running their CDP, Phono pre and Tuner with these tubes.

Any further thoughts?

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.