I tried reading and understanding most of the posts, but confess not
understanding all of them.
BUT, I cannot resist inserting my "two cents" worth:

In olden days, it was quite common (but unwelcome) to have parasitic
oscillations run away in large RF amplifiers. They would start all by
themselves and quickly begin roasting tubes, coils, even adjacent conductive
surfaces, whatever got in the way. The often encountered case would be a HF
amplifier running away (self-oscillating for the unwashed) at a VHF or UHF
frequency.

One common, but mysterious result was burned parts of RF chokes in the
plate circuit. (you do remember tubes, right?) I think there were a couple
of commercial RF chokes wound on long ceramic (steatite?) cores with
assorted winding spacings on the ceramic core. Burned spots would occur in
only one or two places on the length of the winding. Caused, supposedly by a
high current node on a VHF or UHF resonant portion of the winding.

4 1/2 turns at 600khz must be an awfully low impedance circuit, but
could be right down the alley for VHF.

Brian has my vote for barking up the right tree.

Old Chief Lynn, W7LTQ