kt88 vs. 6550 noobie question

No. Not just plug and play. Bias will have to be adjusted to accomodate 6550's unless your amp has an auto bias curcuit which will compensate for the difference.

But, that is not a big deal because even new/different KT88's, when you replace yours, will need to be rebiased as well (unless your manufacturer has advised you to the contrary in its manual). They all have different bias points.

FWIW, whether or not any KT88, or a specific KT88, sounds better than a 6550 depends on the design of the amp (as well as your own sonic preferences). I've used SED KT88's and SED 6550's because I not only like their respective sonic signatures but their ruggedness which allows me to use them in some pretty demanding amps (which eat a lot of other tubes from both China and Russia).

In some mono's designed to use KT90's, I use the KT88 which adds a bit of warmth in the lower mid-range. Conversely the KT90's and the 6550's are a bit lacking in body.

In another amp, which was designed for KT88's, I found the SED KT88's a bit too thick in the lower mid range and unresolving. Substituting the SED 6550's produced a perfectly balanced and resonably resolved sound.

While I have no issues with Rodman's advise, I would suggest that (in addition to Kevin Deal perhaps) you contact the manufacturer before substituting tube types not indorsed by the manufacturer in its literature and find out if there are any limitations or specific demands their amps make on tube types as well as brands. Especially if your amp has high plate voltage which will often stress the hell out of some tubes and cause pre-mature and major failures which could cause damage to the amps themselves.

Be somewhat careful folks. There is a learning curve with tubes. :-)