bad alt or bad VR?

Yesterday while driving my 73 dart(/6car) i noticed the alt guage started discharging. when i got home i ran a test with multi meter, same results with car running and not 12v so i threw a spare alt in. when i was running it after the guage was running high, and would peak when i revved it.
my question is...is this normal because the new alt is charging the low battery after driving with a dead alt?
also what would i see on the guage if the VR were bad?