I checked my battery voltage at 12.6 V. The power side of the ballast resistor was 11.6 V with the key on. The distributor side of the ballast resistor dropped from 11.6 V to 5.7 V with the key in the run position. Is this voltage drop in run correct? It seems a little low to me as it might create a low voltage spark.

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225 slant six with automatic transmission.

What type of vehicle are we talking, year, stock charging/ignition systems?

You have a voltage drop problem in ignition circuit that needs to be corrected, and low battery voltage. Your voltage regulator is probably see 11.6 volts as well and upping alternator out put an additional 1 or 1.x volts to make up the difference to bring charging circuit into normal range.

First make sure battery is fully charged to around 12.8 to 13 volts.

Second, clean all battery connections grounding conductor connections at block, head to fire wall, and brass connections at bulkhead connector and ignition switch. Keep in mind ground path of electron flow is as important as electron flow through wires. Recheck voltages along circuit looking for drop and replace or re-clean green crusty brass connections and or a bad conductors as needed.

With ignition in "ON" position check voltage drop between voltage regulator's base or chassis by placing one VOM prob on voltage regulator chassis and the other on battery post not clamp repeat same for orange box if equipped, and alternator case. Reading needs to be zero to less than 0.1 volt. If you find voltage drop re-clean bolts attaching VR & orange box to fire wall to reduce resistance in ground path, if this won't correct the problem make up a ground loop that connects to alternator's case, electronic ignition orange box chassis, voltage regulator's chassis, and back to negative battery terminal. This will zero out ant voltage drop.

One more thing:
If you have an electric choke disconnect it, and check voltages at ballast resistor. Depending on how choke circuit is configured it may be in parallel with ignition circuit dropping ignition circuit's voltage. The fix is to use ignition circuit to trigger a relay to feed choke directly from battery. Trigger side of relay takes very little amperage unlike choke's high resistance bi-metal spring and won't lower harness voltage.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC