For those that haven't seen an original ammeter, I'm posting this link from Vince's site.
http://www.fordgarage.com/pages/ammetertool.htm
The originals were not fancy by any means but the needle movement was consistent and much more sensitive than any repro I've yet seen.
A person working on the 3-brush type power generating systems needs to think in amps and not worry so much about volts. The voltage is controlled by the battery so it's very important that the battery be in good condition and well connected. When you adjust the 3rd brush, you are adjusting output amperage. Voltage is basically along for the ride in terms of voltage values.
A generator has to be able to generate a current/voltage by the amount of residual magnetism in the field pole shoes to even come on line since that is what closes the cut out switch. Once it's on line then it has the battery to control the voltage output and the 3rd brush to control the amperage/current output. The only bad thing about the original system is that it has to put out a constant current to keep up with the load. There is no rest for the weary and battery can be overcharged by the system if the load is low with high output current. It's not all that good for the generator either. If a person runs the car during the day then the 3rd brush can be set low but if you run at night a lot then it has to be adjusted back up again. This practice is not always followed that well. If you run with the lights a lot and don't want to adjust the output all the time then you have to run with the lights on all the time.
I've seen repro plastic terminal boxes melted due to loose wiring in there. Poor connections equal high resistance in the system and high resistance equals high temperature of the problem area. The wiring and ground pathways have to be maintained in good clean condition with tight connections to minimize problems.