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.

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.