No power at all

i recently bought a 72 Scamp with a 360. A few times I’ve gone to start it and there is no power whatsoever. At first when I get in there’s domelight and dash lights but when I turn the key all power shuts off no crank or anything. It will jumpstart just fine.
Lets break that up.
If it starts up with a jumper, the key switch should be in RUN.
When you turn the key and all power shuts off, is this in Run, or Start?
Also what does the ammeter show in each of these cases?