Why do fuses blow in guitar amps? According to our friends at Mesa/Boogie, the reason is almost always a bad power tube or rectifier tube (assuming there wasn’t a major power surge). A bad preamp tube will not cause a fuse to blow, because preamp tubes are not high-current components.
Keep in mind that — again, assuming a power surge has been ruled out — replacing a fuse does not fix the problem. The solution is to replace the fuse then begin by substituting a known-good rectifier tube (if your amp has a tube rectifier), trying the amp, and if that doesn’t solve the problem, then move on to substituting known-good power tubes.
Note that it’s rare for more than one tube to fail at the same time, so you’re generally trying to find the one bad apple in the bunch.
The moral of the story: always keep spare tubes and fuses close at hand!