In Reply to: Re: Charging system 51 8N 6V +Gnd posted by Clarke on January 14, 2020 at 10:07:52:
The B at the end of the part number indicates a revision in design or construction of the part.
In an A circuit charging system the regulating resistor is between the output end of the field coil and ground. It grounds in the regulator case.
In a B circuit charging system the regulating resistor is on input side of field coil and output side of coil is grounded to generator case.