[julie_m]

13.47V off-load wouldn't be unusual for an unregulated 9V supply using an old-fashioned transformer, especially if it was going to be expected to produce an amp or so. Measure the voltage under load and see how the voltage sags. If in doubt, replace the PSU with a regulated one of the same voltage and polarity and good for at least as much current as the original.

As far as selecting a replacement fuse goes, just measure the current drawn by the set with the volume turned up loud, and fit the next rating down. Ordinary glass fuses aren't even going to think about blowing till they see about twice the rated current.

It's unlikely for a faulty PSU to blow a fuse, because a set with a voltage regulator won't draw any extra current at higher voltage. But it's also possible that there is some sort of last-line-of-defence protection circuit which shorts out the supply and blows the fuse in the event of an overvoltage. You could test for the presence of such a circuit using a current-limited bench power supply, if you felt so inclined, but do not leave it in the protection-active state for any length of time: in real life, it only has to last long enough for the fuse to blow!