As RR said, you have to run a regulator to control the alternator output, putting 12V to the field stud of the alternator is for testing purposes only, you are full fielding (max output all the time) if you leave 12V hooked up the the field terminal on the alternator.

In the stock, original setup the 12V to the alternator field terminal is cycled on and off by the regulator, that's the black box to the driver's side of your master cylinder in the pic above. There are two wires to that regulator, one is switched 12V, should be there when the key is in run and should be very close to, if not identical, to battery voltage. The other wire does to the aforementioned filed stud on the alternator. The original regulator is mechanical in nature, it turns on and off cycling 12V to the field terminal. Internal to the regulator connected to the IGN terminal is a fusible link (looks like a strand of wire) I have seen them pop and open rendering the regulator dead. If you have nothing coming out the FLD terminal (this is the terminal on the regulator that is hooked tot he alternator field terminal) and you do have the switched 12V on the IGN terminal of the regulator then you can replace the regulator and should be good to go.