What both Mike, and Lt Bob said are right on the money,....
and let me add this,....
DO NOT MESS WITH ANYTHING INSIDE YOUR AMP UNLESS YOU KNOW WHAT YOU ARE DOING!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
there are LETHAL voltages runnin around in there, and if you do not know EXACTLY how to test, or adjust any of those parts,.... DO NOT ATTEMPT TO DO IT YOURSELF.....
That warning having been given,......
Changing power tubes is absoluely not a hard thing to do,.....
I, like Lt Bob.... have been around tube amps for many years,... and I have never bothered to re-bias any of the amps I have, or used to have,....
what bias does, is to adjust the voltage going to certain parts of the power tube structure.... running hot, gives you a dirtier sound, but will burn thru your power tubes in short order...... running cold will also wear them out sooner than normal,..... so having the PROPER voltage to the plates gives you the MOST for your money in terms of tube life,.....
after saying that,..... just realize that to just change tubes to another set of the same,.... you should be good to go,....
Most 'SETS' of tubes, are matched as close as possible to each other,.... but will be different sounding than say another set from a different maker....
and you do not have to stick with 'MESA' branded tubes, they are just branded that way to meet what Mesa thinks is good for their amps.
It does NOT mean you have to keep those in your amp.....
you can use groove tubes,.. or JJs, or Svetlanas, or whatever,....
They will all sound a little bit different from one another due to manufacturing differences......
the guy at Eurotubes is real good about matching up your style of playing,.... your amps tubes, and with a short conversation, can recommend several options for your particular situation.
just google Vacuum Tubes, and you will have plenty of stuff to study up on.....
otherwise,..... have fun..!!!and don't worry so much about the bias, just stick a new set in there and play...!!!!!!!!
Steve