According to Scott Willing :
I fished out a good old-fashioned analog meter, and using that I
measured 14V steady as a rock throughout the operating range. I was
quite impressed frankly; I expected more of a stepped response. Helps
that the battery is in good shape I suppose.

Yeah. A good battery will generally not vary much at all.

I'd love to analyse in detail how this particular voltage regulator
works. Replacements are still available but as an electronics guy I'd
be tempted to build myself a bullet-proof solid-state replacement.

Are you getting the tractor maintenance manuals? If not, I could probably
scan the pages from mine.

Perhaps I can figure out how it works from the manual.

I was told by a good small-engines guy: "Don't buy a replacement regulator.
If it fails, just about any generic (solid state or otherwise) regulator
will work fine, and cost MUCH less."
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.