"Tim Shoppa" wrote in message
ups.com...
John Popelish wrote:
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.

Wow, man, if I make it happen again I see the screen voltage
motorboating up and down by about 3V (around the nominal 150V) at about
30kc.

Changing the screen bypass capacitor between
680pf/0.001/0.002/0.005/0.010 and changing the current through the 0A2
between 5mA and 10mA and 20mA and 30mA doesn't stop the squegging but
it does somewhat alter the timing/amplitude.

Plate can be held at a steadyish 350V (even bypassed)
through all this.

So this is something like the textbook squegging which seems to be
something like a motorboating of the plate voltage, but in my case I
see it in the nominally regulated screen instead. New one for me!

I don't think this is quite like the typical NE-2 relaxation oscillator
circuit, because I thought 0A2's were supposed to be stable with these
small amounts of capacitance and the behavior seems independent of room
lighting, but I could be wrong. The screen voltage waveform sure as
hell looks like a relaxation oscillator at 30kc.

Looking at my old schematics I see my Heath HW-16 crystal oscillator
puts the crystal between the screen and the grid of a 6CL6. Manual says
that the screen is serving as the plate of the oscillator. Probably
completely unrelated to the intended operation of my oscillator (where
I bypass the screen and the leads are short) but may be related to the
unintended mode of operation!

Tim.

I think you are talking yourself out of the problem.

It sounds like you are trying to build an electron-coupled Hartley
oscillator. There is no way that a voltage change of 3 volts on the screen
(which is functioning as the anode, for Hartley purposes) is going to start
and stop the oscillator.

Watch the DC voltage on the grid. You will see it follow the relaxation
oscillator waveform when the thing is squegging.

The fundamental problem is two-fold. First, too much feedback. Second, the
time constant of the grid circuit is too long. The stored energy in the tank
can continue to charge the coupling cap even after the plate current is cut
off, and it can't start to oscillate again until the charge on the coupling
cap bleeds off.

Start by lowering the grid circuit resistance and lowering the capacitance
of the grid coupling capacitor. This will work to cure both evils. Taking a
quick look at the 6AH6 curves I would guess that, at 6 MHz, your coupling
cap should be about 22pF and the grid return about 10K. The 6AH6 is also a
sharp-cutoff pentode, so it will be very sensitive to small grid voltage
changes near cut-off.

Don't get distracted by the swings in screen volts, you are just drawing
large pulses of current and pulling the supply down. Unless the 0A2 is going
out of conduction RC oscillation is unlikely.