EL34 in a 6550 amp?

The amp would need a little modding to do so.

Yes you can use el34

Well, you've received a no and yes so far.

What does the manual say about that? There are some amps that can use all 3 types, but if it's designed to get all there is out of an '88, probably it'll fry a '34.

Some amps that take EL 34s and can also accomodate KT-88s KT-90s KT77s and of course the 6CA7. Some will tell you that even the auto biased amps require some manual biasing to do this. It certainly makes intuitive sense that different bias voltages to the screens and plates for each tube would be required for them to operate optimally.
None the less I use KT-88s in my Jadis DA 60 which originally came to me used with EL 34s. I am not certain what it was built with however. I was informed KT 88s would work. They sound fabulous and are my favorite in this amp (GL reissues).
My thinking though was that transformers were more or less tube specific but I could just be running them at a lower plate voltage or something?? than is optimal. The bias is automatic and hard to set manually I have been told.
The only tube I have not rolled into this pin out is the KT-66. I am afraid to try that but understand that this whole family of power tubes was derived from that one.
Please correct me if I am wrong.

You may get more from this:
http://www.thegearpage.net/board/archive/index.php/t-214517.html