Tim Shoppa wrote:
John Popelish wrote:
The grid acts as a rectifier that builds a DC grid bias
voltage from the rectified AC signal, biasing the tube off.
Then the tank dies a natural death.

But in a "normal-functioning" oscillator, the DC grid bias doesn't cut
things off for so long, right?

Right. If you don't have way too much loop gain, the bias
just shifts enough from the grid leak effect to slightly
lower the loop gain so that a stable oscillation takes
place. The negative feedback loop that adjusts this gain
adjustment effect can be stable or unstable.

The P-P amplitude at the grid (as seen by my 10x scope probe) when the
circuit is not squegging is in fact larger than when it is squegging.

I suppose it is possible there's some weird kink in tube
characteristics for all the pentodes I tried.

Adding the series
resistor reduced the efficiency of the rectification.

Sounds like you have too much positive feedback, to begin with.

Probably, but moving the tap on the tank coil had little effect. The
handbook says about a third of the way up from the ground end, but I
tried it at a half, two-thirds, one-tenth, etc. It did alter the shape
and timing of the squegging a little bit but it was still squegging. If
I moved it too far the circuit didn't oscillate at all (too little
feedback).

Also, changing the biasing (trying to move it further into class A) by
putting a cathode resistor in didn't help much either.

My concept may be over simplified, and not include
everything that is happening. I would look at the screen
bias voltage during the squeeging to see if it is also
bouncing with it, or remains stable through a cycle.