trinityamps.com

The build diagram shows the 6V6 as down and the 6L6 as up on the bias switch. I am using a KT66 which required no mods to the stock build. When I left Amp Camp we had the switch in the down position (towards the bottom of chassis) and the KT66 worked just fine. If I am substituting the 6V6 do I need to switch the bias? I ask this because I tried the 6V6 and saw the tube heating very quickly (perhaps too quickly). Also there was some noise so I shut down quickly with no damage done. That left me wondering if the KT66 and 6V6 use the same bias position.

Yes the bias switch is for either a 6V6 or 6L6(Kt66). IOW, yes you must throw the switch when changing from 6v6 to 6l6(Kt66).

What happens if you mix up the positions? I assume the KT66 would run cold and sound like crap and the 6V6 would run hot and blow up. Any damage to components other than the tubes?

That the 6V6 will run hot and possibly red plate and the KT will run cold.
Un plug the amp and let the B+ die off. Pull the o/p tube out and put the + lead of you ohm meter into pin #8 and the other lead to ground and after letting the big cap charge up you should get a ohms reading of about 750 ohms for the 6V6 setting and about 325 Ohms for the 6l6 setting.
Stew

So to be clear: when behind the amp pushing the switch towards the speaker is 6V6 and back towards yourself 6L6/KT66 correct?