Here are a few pictures of a Plexi 100 watt head that I built using parts from Ceriatone.  My transformers came from AES and the choke is a Classic Tone.  The guitarist that uses this amp liked the sound quite a bit with a set of JJ E34L's but he LOVED it with the KT-66's which he's been using for a long time in a 50 watt Marshall. 

The favorite KT66 for us had been the Saratov piece that was sold by Groove Tubes as the KT66HP.  They got tough to get so we switched to using the Valve Art.  Now New Sensor has a KT66 out that's very similar to the Saratov version and it's sold as the Genelex KT66. 

Using this particular tube in this circuit is not difficult but it's not plug and play either.  The bias pot's range needs to be changed so that you can get enough negative bias for proper operation. 

What I did was replace the two bias "splitter" resistors which are 220k in the stock amp with two 100k resistors.  Then I added another 22k resistor in parallel with the existing 27k bias resistor.  I did this because it made it easy to fine tune the range on the bias pot.  Replacing the resistor with another value would be easy too. 

In my amp I have 485 volts B+ and the tubes seem to need about -60 volts to get them to idle at about 30ma current.  That's all there is to it.  Other KT66s or 6L6's might require some tweaking of these values but they will probably get you close.

You also will have to change from the "bear trap" tube holders to something else.  I just used some I had on hand but I had to stretch the springs a little bit to allow them to fit the chunky KT66.

Using the KT66 in this circuit is not a purist thing to do I guess but it opens up some cool sounds.  Myles the guitar player who uses this amp plays mostly rhythm guitar and he likes to use rather clean punch tones and the KT66 is a good tube for this because it gives a mixture of the EL34 crunch with the 6L6 punch.