Tim Shoppa wrote:
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.

What are the three terminals of your oscillator? Your screen is
grounded, so I assume that the grid and cathode are both floating at RF
-- is this so?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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Tim Wescott wrote:
What are the three terminals of your oscillator? Your screen is
grounded, so I assume that the grid and cathode are both floating at RF
-- is this so?

The circuit is the "tuned plate Hartley" as it appeared in any
50's/60's/70's ARRL handbook.

The grid tuned circuit is at 1.8Mc, the bottom end of the inductor is
grounded, there's a tap nominally one third of the way up to the
cathode, and the top of the inductor is connected via a 100pF capacitor
to the grid, which has a 47K to ground.

The plate circuit in my current incarnation is tuned to the harmonic at
3.6Mc. The "untuned plate" version in the handbook has a RF choke
instead of a plate tuned circuit, and indeed I had this on the bench
for a little while.

Tim.