It's unlikely to be a valve blowing the fuse, more likely the rectifier (that's probably a silicon diode or bridge) has failed, or the mains transformer is shorted turns.
Repeatedly putting fuses in and blowing them won't have helped anything, as will fitting a slow blow fuse in place of a fast blow one - assuming the rectifier was short, it might have now killed the transformer.
But valve amps are crude, simple, and easy to repair - but you do need some knowledge about them.