"The oldest Onans used no voltage regulation at all. The faster they spun, the more the output voltage."
I beg your pardon. The voltage REGULATION control process they used is referred to as "saturated field". As long as the Hz was maintained near 60, the voltage was great. So was the waveform. The problem of these older Onans was speed control. This is where me and Woodward Controls made great friends. Even 30 years ago I was getting 60.0 + - .3 Hz speed control. No these generators did not use a Voltage Regulator with ready-to-burn transistors, diodes, and resistors. Basler regulators were so bad back then half the trawlers switched over to saturated field generators.
Below is a superb PDF explanation of the process, also known as "self-regulation"
http://web.uettaxila.edu.pk/CMS/SP2012/etEMbs/notes%5CDC%20Generator%20x-tics.pdf
