| Hate to say it, but this is one of those "if you have to ask, then it's tech time" -type problems. (Absolutely no offense intended.) You could unplug the amp and open it up and make sure all wires with connectors are in still place, nothing touching the case metal where it's not supposed to, no wire insulation worn off, no loose board screws, no screws floating around inside & shorting things out, i.e. all the common-sense stuff, but beyond that there really isn't much you can do. You MIGHT try removing the preamp tube(s) and see if the fuse still blows, which would eliminate a shorted tube, except I hesitate to run an amp absent even one of its parts when I don't have a schematic for it to predict its effects elsewhere, so please wait for a second opinion on that before you proceed.
Was the amp used hard last time or did this fuse-blowing come out of nowhere? | 
