pri mains i ass sume, not B+ sec fuse for backup?
this is scary your first time, but no biggy with low power.
disconnect the xfmr sec leads that go to the rectifier bridge.
short out the transformer leads, thats right, connect them with a wire nut, plug it in, measure pri ac current, s/b around 100 ma or so, depending on how many 312's you have chained.
just make the fuse so that when the xfmr is shorted. the fuse blows
so ie 100 ma draw, make fuse 50 ma slow blow.
guess what happens if you make the fuse 120 ma so it wont ever pop?
thats right, varnish in the morning, varnish in the evening, aint we got fun!
cj