How did they explain "current flow"?  We received seemingly interminable
    descriptions of how the filament heated the cathode ... this heat
    "boiled off" electrons which were attracted to the plate (after dodging
    the various grids) and returned to the positive side of the battery
    ... and the electrons that were lost from the cathode were replaced by
    electrons from the negative side of the battery.

Well, it was a pretty theoretical program.  IIRC, they said something like
"traditionally, current flow is the opposite direction from electron flow,
but that's just a mathematical artifact."  When explaining PHYSICS at a
partical level (like your tube example), we'd get a story like yours, along
with "and of course that cause a current flow in the opposite direction."


    In present day terms, does a "source" provide conventional current
    or does it provide electrons?  Same for a "drain" ... does it drain
    conventional current or drain electrons?

Um, I don't actually recall.  Most of our FETs were theoretical and fairly
symetric, too, I think.  I don't recall ANYONE ever justifying the names of
semiconductor terminals.  Hmm.  I can dig "Solid State Electronic Devices"
(Streetman) off my shelf, and lookup the chapter where FETs are
introduced:

        "In figure 8-8, electrons in the n-type channel drift from right to
        left, opposite to the current flow.  The end of the channel from
        which the electrons flow is called the source, and the end toward
        which they flow is called the drain.  The p+ regions are called
        gates.  If the channel were p-type, holes would flow from the
        source to the drain, and the gate regions would be n+."

So I guess the terminals are named based on the actual charge carriers
rather than either current OR electron flow, but it doesn't SAY that.

BillW