OK, my mistake- - - -I thought you were dealing with a generator system that has 3 sets of contacts inside the regulator. You've got the early-style external-regulated alternator. That regulator has a set of field relay points that will be open with the engine off, and a set of spring-closed voltage regulator points. The spring-closed points are the ones that need adjusting. Don't mess with the field relay- - - -it's either good or bad, and won't need adjusting. Slightly reduce the spring tension on the VR points, run the engine at fast idle,, and look for 14.5-14.7 Volts at the battery posts with a multimeter. If you want to check the alternator output, turn on all the accessories you can, like high beam headlights, wipers, heater fan on high speed, cigar lighter, etc., and check the battery voltage at fast idle under full load. Anything above 12.5-13 is OK with a big load applied.
I looked up some potentiometers on Ebay- - - -one heavy duty enough to adjust battery charger output is going to cost somewhere in the $30.00-$50.00 range, so a dedicated 6V trickle charger will be a better choice. Keep a close watch on the water level of the battery when it's on a trickle charger- - -you don't want to let it get low. No, a household light dimmer probably wouldn't work well for that application.
Jerry
