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A few possibilities... 1 - A bad (high resistance) ground at the alternator as suggested below. 2 - An intermittant connection somewhere in the charging circuit. Not just between the alternator and ammeter but also between the ammeter and the battery + post, or the battery - post and the tractor frame. It takes an open circuit for only an instant to fry the diodes. 3 - An intermittant dead short in the charging circuit (unlikely since it would be almost instantly accompanied by smoke, then flame!) 4 - A bad regulator in the alternator. Does the ammeter show full scale, the alternator get hot to the touch, or does the battery "boil" after running a few minutes? If so the regulator causing is running the voltage up too high and this is frying the diodes. If you have an accurate voltmeter, stick it across the battery with the engine running - a reading of 15 volts or more indicates overcharging.
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