check ground wire, make sure it get good ground from Alt to ground and from engine block to ground.
double check your "new" alternator. The voltage regulator is built-in. Perhaps you have one with a defective voltage regulator. Probably from a cheap auto parts store.
Alternator belt tight? Check that too.
Ignition switch to alt 12v, is it present at the alt terminal.
simple 5 minute diagnostic. Good day..