cause the key is on.
If you are only getting 12vdc at the output stud of the alt, and reading 12.5 at the battery, then the alt is no longer charging. Now it may be that its defective/broken or it may be that the field circuit is not working.
"with just key on, I get 12.5 at batt... ballast l is 11.6 both terminals, .. at the VR I get 11.6 out of both the green and blue wire there. why would I get any reading at the green field wire at the VR ?"
Because that is a series circuit. The battery voltage is coming through the ig sw RUN down the blue wire to the VR,to the alt field coil,out the coil to the green wire ,and back to the VR.There will be a voltage drop from end to end, but its all one long string.
In a perfect system, with the engine not running, there would be the same battery voltage every where throughout the system.(except for certain dash gauges.)
In a perfect system, with the engine running, there would be regulated alternator output every where throughout the system.
The battery voltage is very nearly always below the VR set-point voltage. So right after you start the car,and the alt is fired up by the ig sw through the VR, it begins to pump out DCvoltage. It pumps it out hard, every where, and it only takes a moment to get to the VR down the blue wire.The VR says whoa there buddy, thats too much.It then immediately starts leaking what ever voltage is higher than the set-point.It leaks it from the green wire to ground.As soon as the voltage drops below the set-point the VR stops leaking, and the blue wire voltage rises. Thus it regulates the system voltage by varying the field leakage.The VR set point can be anywhere from a low of about 13.2 to a high of about 15.2. It varies by manufacturer.As a very general rule, on our classics,the charge voltage runs about 13.2 to maybe as high as 13.8. And thats about what you should be seeing everywhere except on the coil side of the B-resistor.Here it should start out near that same reference voltage, when its cold, and the voltage should drop as it heats up. Which doesnt take long.
So thats why the VR case has to be well grounded; so it can leak. And why the alt has to put out DC voltage, so the VR knows whats going on. And why the VR needs to see battery voltage at the blue wire.
High resistance connections anywhere in the system between the battery and the alternator, can fool the VR into thinking it ought to leak less and therefore puts out more voltage.
Do you know how to perform a full-field test? You will need to perform one on your alt to prove its good or not. Also you will need to check it for A/C output.