Can an EL34 tube be replaced with a KT88 or 6550?

All EL34's don't sound alike, not by a long shot, so even if you can't use 6550's and KT88's, you can get some tubes which will sound significantly different. You will still need an auto bias curcuit or the ability to re-bias the amp with the new tubes. Ditto small tubes except you don't need to re-bias for them. Good luck......

Smurfmand70, One of the nice things about EL34's is they are relatively cheap and 'just getting them all' is not a bad idea. However there are other things to consider in selecting power tubes, one of which is thier ruggedness and how your amp will deal with a shorting power tube.

For example if your tube is fused in a manner that protects its circutry when power tubes short out you simply have to replace the tube and a fuse, however many amps are not made that way and a shorted power tube will take out a resistor and unless you know how to repair it (a small thing really) you're going to have to haul your amp to the audio store for repairs. In that case your priority might be or include ruggedness.

Also, each brand of power tube, as much as different types, have a generic sonic signature. You can discover this signature by doing a lot of reasearch on the 'net. Having done that, AND having assessed the tonal needs you have in mind, you might not need to try them all to find a good one.

Two last comments. 1) Include SED EL34's on your list of maybe's. They are rugged and they sound great in many systems. 2)Unless you are really anal and have a lot of money, you can really do well with new production power tubes. I can think of few reasons why one needs NOS power tubes to get great sounds with most amps.

Have fun............