A couple of tips when changing tubes:
-Make sure your amp is off (I've heard that you can change out 12AX7 tubes with the amp on standby, but it doesn't take that long to change out tubes, and I wouldn't want to turn my mesa into a $1200 doorstop by mistake)
-If you are removing the 6L6/EL34's make sure they are cool enough to remove (or use a cloth or something because these guys get hot!)
-When removing them, gently rock them back and forth while pulling them out - be gentle
-If you are putting in your 6L6's and EL34's, make sure you line up the slot on the socket with the tab on the bottom of the tube - then gently push in (I also rock them back and forth while pushing down)
-If you are putting in 12ax7's, have patience while lining up the pins (there's a gap in the ring of pins that needs to be aligned with the gap in the socket, and you can't see the gap when putting them in). The pins are delicate and can bend easily, so be patient while you line them up by feel. If you are replaceing with the same brand, I usually see where the logo is facing when I pull one out so I can line up the new one in the same way. If you start pushing it down and it's not going in easily, pull it back out and make sure you're not bending pins.
I've never had a problem getting shocked before while changing them - just make sure you don't poke around inside with anything metal and you should be okay.
That's it (or as much as I know about it) - it's really pretty easy.
Good luck!
