Which mains fuse...?

[acorkos]

Replacement power transformer for a Twin Reverb (see attached pdf)...which mains fuse would be specified?

[Jazz P Bass]

That is a very 'open ended' question, No?

It depends how much current you 'pull' through it.

That said: a typical Fender Twin Reverb will use a 3 amp fuse.

The way to properly size the fuse is to see how much current is flowing though the mains circuit at full power.
Then round it up to the nearest whole number value.