BIAS SET 

There's a lot of confusion about what biasing an amp is or even if its necessary.  While some amps do not have an adjustable bias, or are cathode biased (a kind of self-adjusting bias), most amps need to be re-biased when new power tubes are installed (not necessary when replacing pre-amp tubes).  This is because there's a lot of variation in tubes, and without a re-bias, the amp may not sound or feel its best, and could very well destroy the new tubes and some other valuable parts (output transformers, etc).  

Biasing an amp, in the most basic sense, sets the operating characteristics for the power tubes so that they work as they should in the amplifier.  There is no single, correct number used to bias an amp.  This is where the “tone and feel” thing comes in.  There is a wide range of values you can set bias at such that the amp will power up, make your guitar loud, and be operating ‘in spec’.   That doesn’t mean its operating at its best!  Just setting every class AB amp at 70% of plate dissipation at idle is the ‘looks good on the scope, must be good’ philosophy.  But some amps do not sound or feel their best at 70%, and the more heat the plates are dissipating, the shorter the life of the tube.  The trick is to find the ‘coolest’ bias you can while still giving the amp its best tone and feel.  Some amps like 70% or very close to it.  Some like 50%!  You have to be able to hear AND feel it.  That’s where being an experienced player as well as a tech helps.

Bias is especially important nowadays, when there is a great deal of variation in tubes and they aren’t nearly as robust as they once were.  Back when you could buy good American made tubes, you could probably toss a new set in without setting the bias, and while the amp might not sound its best it probably wouldn’t be in danger.  Those days are gone, there are no American or Western European tube manufacturers left.  I’ve seen many cases now where a set of the same tube type from the same manufacturer will red-plate quickly if just tossed in the amp without re-bias (red-plating is what happens when the plate, a part of the tube, gets so hit it turns red.  This will lead to failure of the tube, and can take out other far more expensive parts).  

Once bias is set properly, you can change tubes without a re-bias as long as you get tubes rated the same as the ones you are replacing.