A healthy alternator, with the VR bypassed and with a good battery that is nearly charged, should be able to ramp up system voltage--depending on engine RPM to dangerous levels. DO NOT DISCOUNT something REALLY basic like a pulley that is polished up from slipping and therefore almost impossible NOT to slip
In other words, after starting, running ?? a minute or two to get the battery back "up" from starting, if you remove green wire and ground that alternator terminal, if that doesn't work, leave that field terminal grounded, then remove the blue field wire, and jumper power to the last field terminal direct from, say, the starter relay stud, then manually raising engine RPM, system voltage should climb right up and I would be careful to hold it below 15.5--16 at most. if it will not do that, concentrate on
dead or defective battery
Measure drop between alternator output and battery
Highly suspect alternator
or the mentioned belt/ pulley / tension slip etc.