When buying power tubes for your amp it is always a good idea to purchase an extra set that is matched to the set you are buying. For example if you are buying a matched pair of tubes for your amp get a matched quartet. Once you set the bias for the first pair from the quartet the second pair do not need to have a bias adjustment. This can apply for amps with any number of power tubes. If your amp uses a quartet purchase two quartets that match each other. This is very helpful if you are on a gig and a power tube goes bad. You can just pull the bad set and put in the new pair without the bias adjustment that could take too much time during a performance. Having a leather glove or something to grip the hot tube is also a good idea if this needs to be done during a performance.