HI, If this is a tube amp, you could pull the tubes and see if it still blows the fuse. If not, get some new tubes. Power tubes tend to arc and blow fuses, but I would go ahead and get a complete set. Sometimes you can see which tube it is by a bright flash of light inside the tube just before the fuse blows.
If it is a solid state amp, or if it still blows the fuse with the tubes out, it probably has a short in the power supply or output stage, such as a rectifier or capacitor. You will need to take in for service.
If it is a solid state amp, or if it still blows the fuse with the tubes out, it probably has a short in the power supply or output stage, such as a rectifier or capacitor. You will need to take in for service.