Ignition problem

I have a 1956 240 utility tractor which I love to death. Last Winter I changed it from a 6 volt to a 12 volt system. I have a problem right now. When I was using a bushhog and putting it under a load, the engine starting running very rough and quit. I found out that the Ballist resisitor that I had installed had cracked from the vibration. I put in new points and condenser previously and then put in a new ballist resisitor in case that the 12 volts coming to the coil had fryed the condenser. After putting in a new ballist resisitor today, the engine ran great for about 10 minutes and the engine died again and would not start. After 10 minutes, it started up again and ran for 10 minutes and died. I felt the coil, and it was warm but not hot. I still have 12 volts coming to the ballist resistor and about 5-6 volts going out to the coil. Also the rotor on the distributor has a smoked, charred film on it.
Do you think the coil could be bad. After recieving 12 volts from the bad ballist resistor, could it be bad. I thought when a coil went, it went bad just like that?
Could the distributor cap and rotor be bad also. Has anyone else had a problem like this.
When I called the dealer, they said that I could use a standard coil on this tractor. It doesn't matter whether it's a 6 or 12 volt system, the new coils work on both. This doesn't sound right to me. Any comments. I would appreciate any wisdom and knowledge. It's such a simple system, but is kicking my behind.

Try replacing the coil. They often fail in such a way as to quit only after running long enough to heat up. What the dealer means is that he has only 12 volt coils. The one he sells you probably doesn"t need a resisitor and will work better without one.

The coil should say something like "12 volt use without external resistor". Or "use with external resistor". Get a coil that is internally ballasted and eliminate the external resistor. I agree and would lean toward the coil being cooked.

Thanks alot, I'll give it a try.

Thanks alot, I'll give it a try.