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I put a 6v alternator on my '38. It's been charging at just under 9v. When I'd turn on the lights, step on the brake or put any draw on the system the ammeter would dip to discharge and immediately snap back to the neutral position. On Friday I was out after dark and when I turned the lights on the ammeter went to discharge and stayed there until I turned the lights off. It then went to full charge and gradually dropped to neutral as the battery recharged. Stepping on the brakes has the same effect now, discharge while the draw is present going to charge afterward. I checked the output and it's now charging at 7.7 volts with no load instead of 9v. I'm thinking the internal regulator is malfunctioning. Am I correct with that diagnosis?

9 volts is WAAAAY too high for a 6V charging system to be producing, whether it's an alternator or a generator. The max voltage against a fully-charged battery needs to be about 7.5 Volts.

Voltage is ony part of the equation- - - -you need to find someone with a high-reading ammeter capable of showing the total current capability. Against a low battery, or with a lot of accessories in operation, the alternator should be able to put out its maximum rated capacity, 50, 60, 70 amps, etc. That's tested by putting a heavy load on the battery terminals with a carbon pile load resistor with the engine at a fast idle, and monitoring the output of the alternator at the main output lead. (the big one at the back of the alternator). If there's a bad diode or two, the voltage will be normal against a fully-charged battery, but there will be less than 1/3 of the rated current capacity available.

The symptoms you're describing are characteristic of diode failure which is knocking out two of the three phases of the stator, and partially blocking the output of the third phase.
Jerry