N9WOS wrote:
You also want to take a careful look at the screen circuit.
It can also act like an output in a parasitic oscillation.
So, try putting a 100, or 1K resistor in series with it too.

Indeed this is a working fix, as I verified on the bench.

It's oscillating cycle.
It starts it's HF oscillation gain cycle.

Actually in my particular case it appears that the UHF parasitic starts
first. But probably just the luck of the bias point.

The signal grows, and the tube bias increases.
The current on the positive peaks grown with it.
The current on the positive peaks get to the point that
the tube breaks into UHF oscillation.
The UHF oscillations are several times larger than the intended oscillation.

Very true here - this is in fact the only time I've seen 400MHz stuff
on my 100MHz scope, so I'm guessing that there is substantial UHF
energy there.

The UHF oscillation only last a fraction of the high frequency cycle.

This part is very true too. Tiny tiny fraction of a microsecond. My
interpretation of the scope trace is that the UHF activity gives the
the HF tank a "kick" and it rings for a while.

Tim.