Battery drains only when car is running... Help

My 70 Super Bee used to burn out one diode in the alt about every three weeks. I used to keep a box of stators/ bridges and alternator halfs in the trunk. Every now and then you could find me off the side of the road "rebuilding" the alt. I got to where I could tell by the ammeter needle and the brightness of the lights, if it had burned one, and got to where it only took about 15-20 min to disassebble/reassemble. I'd take the bridge back home and swap out the bad diode, then put the rebuilt one back in the trunk for next time.

Changed the regulator several times, but never figured out what was causing it.