I upgraded the 55 amp 10-SI alternator in my '65 to a 96 amp 12-SI. I also installed an aftermarket voltmeter and disconnected the factory ammeter. When I start the car, the voltmeter shows about 11 volts and the lights are dim and they stay that way unless I rev the engine to get the tach up close to 3,000 rpm, then the lights get bright and the voltmeter reads close to 14. Once it gets there, it stays there and everything runs fine, even at low rpm. I did not change any of the wiring to the alternator. Does anyone have any ideas about why this happens and how to correct it?