Purdy is correct, doubling the voltage doubles the power output of the generator. The generator is (conservatively) rated at 12 amps. At 6 volts this equals 72 watts; at 12V it's 144W. When 50CP bulbs are installed, current required can run as high as 20A or more. At 12V it's half that, assuming a 12V coil. (A resistor consumes as much energy as the 6V coil.)
Of course there's the hassle of changing all the components to 12V. The easy out is installing a 6V alternator, as I did.
