Remember the Boy Scout's motto? It was "Be prepared". There is no better advice for working guitarists. All players bring extra strings and cords, because they know that they'll break. But what about your amp? Lots of pros bring a backup amp, or at least an extra set of tubes. But for many of us, hauling an extra amp is a pain in the butt, and tubes often fail because there is something else in the amp that is overtaxing them.
I suggest bringing along a simple amp modeler. For years I've packed an old Line-6 POD with my gear. (Actually, it's kept with the soundman's gear. He knows exactly what to do if I give him "the look".) The POD is small and simple and does just fine in a pinch. I keep it programmed with 3 or 4 of the most basic food groups of sound I'll need on a gig. If something were to go awry with my trusty 1960s Fender, the switch out is quick and like they say, "The show must go on".