Guard from what? Hah, it sounds like an asshole-type response, but it's an honest question. If you're playing out with the amp and you're worried about transporting it, you'd do well to get a hard-case for it. If you're more worried about mishaps during a show or the like, you could always get some chicken wire (or something more elegant) and screw it onto the back of the head.
Personally, I had a Triple Rec with like 12 tubes in it and toured the country with it for a year, playing some crazy metal shows and throwing it in the back without a case or anything, and I never had to replace a tube!