If the regulator's gone it'll over voltage when you rev it up and that might blow the fuse by forcing too much current through the fuse that's blowing. The regulator steps down the field current in the alternator to control the system voltage. Otherwise the voltage output is roughly proportional to rpm. e.g. 12v @ 1200 rpm, 24v at 2400 rpm and pop in your case.

thanks that helps heaps so where is the regulator?

If it's original it'll be a box mounted on the passenger side strut wall or near it. Like in this picture http://datsun1200.com/modules/myalbum/photo.php?lid=2298. If you have a multimeter try running it and watching the system voltage (battery terminals shoul do) and see if it goes much over 14V as you slowly bring up the revs.