Despite many old wives tales and incorrect beliefs out there and Billy Bob and Bubbas thinking there's a ballast resistor tucked away and hidden somewhere inside a typical 12 volt circa 40's through 60's tractor coil ,,,,,,Typical coils labeled "12 Volt" or "12 Volt not for use with ballast resistor" or "12 Volt no Ballast Required"
HAVE NO DISCRETE STAND ALONE INTERNAL BALLAST RESISTOR INSIDE THEM (even though Billy Bob and Bubba call them internally ballasted coils)
That being said, on a 12 volt tractor you can use a true 12 volt coil (with NO external ballast required) and such are usually labeled as above.
If you want to use a 6 volt coil on a 12 volt tractor, then you do need the external series voltage dropping (12 to 6) ballast resistor so the coil only sees 6 volts and doesn't overheat.
Many old 12 volt tractor coils (Im NOT talkin high voltage or high energy or high performance or Accell or Mallory automotive racing coils) have a LV primary resistance in the 2.5 to 4 ohm range with 3 ohms often found.
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Today's Featured Article - Third Brush Generators - by Chris Pratt. While I love straightening sheet metal, cleaning, and painting old tractors, I use every excuse to avoid working on the on the electrics. I find the whole process sheer mystery. I have picked up and attempted to read every auto and farm electrics book with no improvement in the situation. They all seem to start with a chapter entitled "Theory of Electricity". After a few paragraphs I usually close the book and go back to banging out dents. A good friend and I were recently discussing our tractor electrical systems when he stated "I figure it all comes back to applying Ohms Law". At this point
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