What does it mean when the Gen light comes on in the 1966 Malibu?

I have a 76' Malibu and almost the whole time I have had it the gen light is on except when I just put a new alternator in it. Then the light would go out for a while, but always came back on after a while. What I found out was there is a Diode that goes bad in the alternator and causes the light to come on. Soon you have to get a new alternator.