Originally Posted by Mr IGH

Plano and Roadie, you're confused because you don't have a proper model for the generator. When driving an LED or Zener load, the generator acts as a nearly constant 500mA current source, independent of output voltage (until the winding saturate, ~12Vrms for the Shimano). The booster/doubler circuit helps increase the generator's output voltage to better match the source to the load. Thevin's equivalence for a non-linear source isn't a valid analysis.