[fixit-p]

The charge light filament is not the regulator for the regulator, besides being a visual indicator of system performance it provides needed resistance in the field circuit. If you don't have a light you can use a resistor. Contrary to some opinions the resistance is not needed to step down voltage for the field (depending on what alt you have the field once excited gets its current directly from the stator winding's). It is there to limit current output from the field terminal thus protecting the diode trio or regulator depending on what type of alt you have.

The diagram below is of a 10SI alt (same for a 12SI) and it shows the diode trio connected directly to the stator, the diode trio is then directly connected to the field terminal. Which we can then conclude that the regulator and field winding's can not only handle full alternator voltage but is designed to do so.



The next image is a test connection of a carbon pile across the battery to test the alternators output. Notice the resistor in the #1 field terminal. If you were to load the battery with the carbon pile the voltage would drop, the sense wire would ramp up the alternator and produce the needed current to maintain voltage at the positive battery post. Without that resistor the alternator is going to push out current through 2 paths 1, the batt terminal on the alt and 2, the field terminal on the alt which is fed through the diode trio (which typically is rated @ 3 amps) shorting out the diode trio in the process. But with the resistor in place the diode trio is protected because resistance limits current flow. The bulb and resistors are also effective in preventing engine run on but the #1 goal of the engineers was to protect the alternator from itself.