There are two kinds of 6 volt alternators, the converted 12 volt alternator and the built from scratch 6 volt alternator. In the first example the 12 volt regulator is removed and replaced with a 6 volt regulator. As we all know by now, when you reduce the voltage output, the amperage output is also reduced. Thus what started out life as a 60 amp 12 volt alternator ends up being a 30 amp 6 volt alternator. Keep in mind that the 30 amp rating only accurs when you are traveling down the highway and not at engine idle. At engine idle the output will likely be about 15 amps.
In the built from scratch 6 volt alternator special wound rotors and stators are used and the alternator is built from scratch as a 6 volt alternator. The result is 30 amps at idle and 60 amps at highway speeds. As you might suspect this type of alternator costs more than the converted 12 volt alternator, but as the old saying goes you get what you pay for...
Randy
