I agree with Bob. A coil designed for 12 volt use has around twice the LV primary winding resistance as a 6 volt coil. Since one of the limiting factors in point ignition systems is the amount of current the points must pass and switch and that amount is typically (older stock farm tractor coils NOT talkin high energy after market) around 3 to 5 amps, that means in a 6 volt coil: R = E/I = 6/4 = 1.5 ohms primary resistance or in a 12 volt coil R = E/I = 12/4 = 3 ohms LV primary winding resistance.
In order that the 12 volt coil have more LV primary winding resistance it might have more turns of wire or wire with a higher resistance, but usually in stock old farm tractors the coil would NOT have any stand alone internal discrete ballast resistor, but instead simply more wire length or turns or higher resistance wire then a 6 volt coil.
Sure, there were a few older cars that indeed had a coil with a compartment inside where a stand alone discrete ballast resistor was tucked away, but that's NOT typical in stock old farm tractors.
Also there can be exceptions to the above, but typical LV primary winding resistances of old stock 6 volt farm tractor coils are 1 to 2+ ohms while many stock old farm tractor 12 volt coils have around 2.5 to 4+ ohms. High energy high voltage after market and performance coils can be different
If you put a 6 volt coil unballasted in a 12 volt tractor it could overheat plus the points burn up sooner (would be switching 8 instead of only 4 amps). If you put only 6 volts on a 12 volt coil the points will last longer and it wouldnt get very hot but the spark would be weak.
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