kt88 vs. 6550 noobie question

Are these tubes interchangable? I.e. can I pull my kt88 output tubes and just plug in some 6550's and it will all just work?
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.
Newbee, hit's the note as you can change out the KT88 with the 6550, but you got to re-adjust the bias. The KT88 are probably better sounding than the 6550, but unfortunately they are a more fragile tube. If you can get a quartet of KT88s past 30 days you're probably all right, but unfortunatley, there does seem to be a high failure rate with the KT88 tube. I'm currently using the current issue of the Tung-Sol 6550, which seems to be good tubes, but true be told the Sovtek 6550 is the standard for realiability.
I have been using KT88s without any failures for some years now. That doesn't mean they are not more fragile than 6550s.
May mean I'm just lucky. The new Genalex Gold Lions KT88s are the best sounding to my ears in my system.
Any time power tubes are changed in an amp(providing it is not auto-biasing, as mentioned): The amp/tubes must be re-biased, then re-checked in a couple days. Give the amp(s) at least 30 minutes of warm up before biasing(to allow the voltages to stabilize). If you are concerned about reliability of a chosen tube: Contact Kevin at Upscale. He'll give you the straight scoop, and his testing/screening/burn-in procedures are second to none. (http://www.upscaleaudio.com/view_category.asp?cat=85) KT-88s (or any other tube) may, or may not, be more likely to fail based on where and by whom they were manufactured, how they were handled in shipment, how they are biased, and if the gear they are used in is especially hard on tubes. Lots of variables. You may be interested in these if your taste is for accuracy rather than tubiness: (http://www.euroaudioteam.com/pdfs/eat_tubes_september_2004.pdf)
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. :-)
In my Cayin A88T I found the SED KT88s to have a somewhat loose, lively, and dynamic more accurate but non-clinical sound. Great bass, good midrange substance, and extended true to life highs with a strong 'front row center' dynamic.

In comparison, I found the SED 6550s in the same system to be a little more euphonic and textured in the central midrange, but slightly less forward than the KT88s in the upper mids. The SED 6550s produced a softer more textured and euphonic midrange than the KT88s, but gave up a little bit of the brassyness in the upper midrange which gives some female singers with an etched voice like Diana Krall and Rickie Lee Jones their distintive voice along with some unavoidable sibilants at the same same. However, although the sound of the 6550s was just a little softer and richer than the sound would likely would in real life, the overall presentation was a little more relaxed and easier to listen to and I noted a distint reduction in sibilants. The tonal shift in sound from the KT88 to a 6550 was a movement toward a slightly less open, and softer mid-hall euphonic sound, but by no means dull. It would be sort of like shifting a little from the sound of a Cary amplifier toward the warmer sound of a older Conrad Johnson amp, although not all the way.

For my system, I ended up preferring the KT88s for its more open vibrant 'front row center' presentation, although both tubes bring something to the table. If I was looking too soften up the sound a bit from a slightly too lively system or listened to classical music or wanted more tonal feeling in the mids for romantic listening, I would go for the SED 6550s, otherwise, if I wanted a more dynamic open sound with good midrange substance, great bass, more accurate tonal representation, and vibrant open top end (without sounding clinical highs like the Genalex Lions, Tung Sol, or EH KT88s), I would go with the SED KT88s.

As a side note, I tried the Genalex Lion KTT88, Tung-Sol KT88s, and some EH KT88s. Both of these tubes have a more linear accurate, yet extended clinical sound to me which many others with good ears prefer. I give the edge to the Genalex Lions over the EH and Tung-Sols in that genre of tube sound, but overall I personally prefer the SED tubes as to me they have more 'body' in the midrange, and although still extended also, they sound more vibrant front row center and less mid hall clinical to me.