On Thu, 04 Jan 2007 08:39:34 -0800, Tim Shoppa wrote:
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.

Check for dynatron oscillations; the screen acts as a plate, sort of.
http://en.wikipedia.org/wiki/Dynatron_oscillator

I can't find a schematic of one, but it's just an oscillator with feedback
from the screen to the grid.

How long are the leads of your screen bypass cap, and where are they
soldered down?

Thanks,
Rich