Denny,
Once a one-wire alternator "kicks-in" at between 600-1200 rpm (a fast idle at start-up does it), it continues to charge at all RPMs. An alternator typically charges in the "+" range at much lower RPMs than a generator does.
I do not use an alternator on my "restored" 6v trucks, they use 6v original generators (and are often in the "-" range when idling, especially with the lights or radio on).
