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A 1N4001 or 1N5401 diode in series with a 10 ohm resistor is what I use. Put the cathode end towards the alternator. Tie the anode to the resistor. Tie the resistor to switched battery voltage. This offers a better solution than any selection of resistors: 1) A 10 ohm resistor ensures a high enough field current to turn the alternator on by 600 RPM or so. 2) The diode completely blocks back-flow current so there is no chance of run-on. 3) For extra credit, you can put a small idiot light in parallel with the 10 ohm resistor. It can even be a little dinky LED that normally would not work for excitation purposes. And if you really want to know what's going on, replace the ammeter with a $12.00 voltmeter. Connect the (+)side of the voltmeter to the output of the ignition switch. Tie the internal light of the voltmeter to this point too. This makes it easy to diagnose the infamous "ignition switch goes bad" problem, and the lighted voltmeter reminds you that the key is on -- should you ever stall the tractor or run out of gas, etc. You can get by with just a diode and no resistor, but the resistor adds a measure of safety and longevity to both the alternator and the diode.
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