SO I was wondering if I wanted to put in a KT88 what modifications would have to be made. I know you will need a bigger power transformer, but is there anything else?

Fender Lite Ash Tele

Satellite 65/T

Yamaha RBX375
So you're replacing a single-ended 6V6 output with a KT88? I'm not sure you can do a direct swap without changing a fair number of things. Here's the data on the 6V6 and some suggested substitutes: http://tdsl.duncanamps.com/show.php?des=6V6 I guess there would be a way to make it work but you'd have to wait till one of the amp nerds get in here..