Gunther, I dont think your tractor had a ballast resistor (sure it could have, never say never, I saw all sorts of modifications in my many years as a used tractor dealer) Its a 12 volt tractor that used a 12 volt coil therefore NO BALLAST REQUIRED. If you used a 12 volt coil PLUS a ballast, the spark would be weak.
On 12 volt tractors (like the later 620 and 720 etc) that used a ballast resistor THATS BECAUSE THEY USED A 6 VOLT COIL and the ballast dropped 6 volts leaving 6 for the 6 volt coil WELL DUHHHHHHHH
NOTE: A 12 volt coil is typically labeled "12 volts" orrrrrrr "12 volts NOT for use with ballast" orrrrrrrrrr "12 volts NO ballast required"
NORE: Theres a misconception out there that some 12 volt old tractor coils have a stand alone ResisTOR inside the can WRONGGGGGGGGGGGGG
Contrary to what many lay persons believe who call them coils that have an "internal ballast resistor" YOU WILL NOTTTTTTTTTTTT FIND A DISCRETE STAND ALONE RESIS"TOR" TUCKED AWAY SOMEWHERE INSIDE THE CAN. The way a "12 volt" coil achieves its necessary 3 or so ohms of LV primary resistANCE is by enough wire coil length (more wire or more coil turns) orrrrrrrrr using wire with certain resistANCE per unit length so the ResistANCE end to end (+ to -) ends up in the range of 2.5 to 4 or so ohms so the points dont burn up and the coil doesnt overheat and handles the current when 12 volts is applied.
SORRY CHARLIE, DONT DISECT ONE AND EXPECT TO FIND A STAND ALONE "RESISTOR" HIDDEN INSIDE
NOTE they actually did make some very early automotive coils that DID HAVE a stand alone discrete ResisTOR in a seperate part of the can!! Many had like a ring where the internal can portions were seperated, coil in one end resisTOR in other end all by itslef!! HOWEVER in alllllll my years as a used tractor dealer (older tractors) I never saw a coil with such an internal resisTOR inside the can!!!!!!!!
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