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The only component of an OEM model A electrical system that deals with voltage is the battery. The 3 or 5 brush generators put out current depending on how the field brush is adjusted. I use positive ground since that is the way it was designed to work. The diode for a cut out is OK but the actual cut out relay is my preference. I set the field brush for 5 amps at idle using an original ammeter. The repro ammeters are not up to my standards.
I would go all 12-volt negative ground if I wanted to go that route. Use a modified GM 10si alternator for charging and higher current function. It's the simplest way to go 12-volt. Change all the bulbs and either ballast the coil or use a 3 Ohm primary type coil.