Overcharging '74 Spider

I have a '74 Spider with an alternator. A new battery went bad rather quickly. While the engine is running, the battery reads just over 13V. Per my brooklands workshop manual, I disconnected the red wire at the alternator and read the voltage between the terminal to which the red wire was connected and ground and read 50volts. I replaced the voltage regualtor and still read 50volts. I placed an ammeter in series between the terminal and the red wire and show approximately 12 amps. The manual says it should be 30 amps. Can anyone shed some light as to what is malfunctioning and perhaps any additonal tests i may run. I guess it could be the alternator but it is too expensive to just replace and see if that cures the problem. Thanks