There are 6 diodes in an alternator, two for each of the phases (only 2 diodes are needed for full-wave rectification when you have a center tap or neutral return).
My alternator failed because the brushes wore out. There is a problem in the design of the dummy light circuit in my car (240D) in that you lose the dummy light when one or both of the brushes lose contact. If you're really observant, you will see that the alternator light doesn't come on when you switch the key on but don't start the car. This should be the clue that you have a problem, but if you're like me, it's easy to miss.
If you have a shorted diode, the battery will be discharged through the alternator when the car is not running. My experience is that the diodes don't fail that often, but you can tell by seeing if the alternator is drawing current from the battery when the car is turned off.
Another tip that I have learned the hard way...never use the cars alternator to charge a completely dead battery, if you have a choice. You are much better off to use a battery charger to bring the battery most of the way back up. Pulling 70 or 80 amps out of any alternator for a period of several hours causes the alternator to get very hot, and heat is the enemy of semiconductor reliability, ie the diodes.
I don't really understand what is meant by a "weaker" alternator. If the voltage regulator, brushes, and coils are in good shape, the alternator will easily put out its rated amps. They really are a very, very good, robust, design.
I hope this wasn't too boring.