OK, this means you have the ISOLATED FIELD alternator, wrongly called "dual field" and your regulator should look like this:
http://www.mopartsracing.com/parts/electronic/reg.gif
A couple of quick checks you can do.
First, pull the GREEN wire off at the alternator, one of two push on terminals
GREEN goes to the regulator,
BLUE comes from the IGN switch and supplies 12V to the other field terminal.
With the GREEN off, hook a clip lead from the alternator terminal you exposed to GROUND
This will cause the alternator to "full field" or go to full output.
Start the engine, slowly bring up RPM while watching the ammeter. If the alternator is good, the ammeter will come up to charge. Don't get carried away, easy on the RPM until you see what the ammeter does. If you have a multimeter, it's a good idea to hook it to the battery, prop the meter up so you can see it and the ammeter. Don't let the voltage climb much over 15.5 volts.
For a rough idea on how much the alternator can put out, you can turn on everything in the car you can, stereo, heater, lights, 4 way flashers, etc. Again, don't over rev the engine.
Gently bring up the RPM until the voltmeter is between 14-15, and the ammeter should be in the charge zone somewhere.
If none of this works out, you have problems in the alternator OR the charging lead circuit is bad
To check this, set the engine on a good fast idle and note the battery voltage. Now move the positive meter probe over to the big output stud on the alternator. There should not be much voltage difference between the battery and the output stud, certainly not more than a volt or two.
If there is a big difference, you have a wiring problem in the charge wire/ ammeter circuit. Your number one suspect is the bulkhead connector.
If nothing seems to work out, maybe the field is not getting voltage. With the engine off/ key in "run" and your clip lead in place to ground, check the BLUE field wire. You should have "close to" battery voltage there. If not, you have a problem with that wire in the switched IGN circuit. Move your meter up to the bulkhead coil ballast and see if the IGN supply is close to battery voltage.
IF the voltage is OK at the ballast, the wire is broke from the ballast down to the BLUE field connection
If not, your switched IGN circuit is messed up, that's another story
If it ch
Now if the above test shows OK, you probably have trouble in the field/ regulator circuit
So remove the regulator connector, and devise a way to jumper across the two terminals in the harness connector. This might take a second person. Again, start the engine, gently raise the RPM while watching the (battery) voltmeter and ammeter. IF you get a good hot charge, you most probably have a bad regulator
If not, you probably have a bad circuit in the regulator / field, and/ or bad connector itself.
Note the regulator plug has a green and a blue. The BLUE just like the BLUE at the alternator field, comes from the IGN switch, and should have "close to battery" with key on
The green should show continuity (zero ohms) from the regulator harness connector down to the push-on terminal at the alternator field.
If all this check out, change the regulator. Make sure it's grounded, and post back with progress or questions.
You have a shop manual/ wiring diagram?
Download a 72 shop manual for free from this thread:
http://www.forabodiesonly.com/mopar/showthread.php?t=132309&highlight=manual,+download
Here are usable diagrams :
http://www.mymopar.com/downloads/1971/71ValiantA.JPG
http://www.mymopar.com/downloads/1971/71ValiantB.JPG
simplified charging system diagram:
http://www.mymopar.com/downloads/Dual_Field_Alternator_Wiring.jpg
See? easy.