there's a resistance unit available from some of the repro places [Filling Station?] called "VOL-TA-DROP" that does the job - about 3" sq ceramic slab w/ a heavy wire wound resistor unit and 2 different current taps, from 5&6amps to 10&20amps - I've got one [10-20] from long ago that I used to run a nice 6V fan in a 12V truck
Bill