Tim,

My guess is the OA2 (Neon) and Cap combination is causing the problem.
Isn't that an oscillator in itself ?

Try replacing the OA2 with a Zener (or 3 in series). It may be a bit
noisy, but it may also be stable.

Or... a pair of resistors to derive 150V from HT. Just to see if squeg.
stability improves/changes. 15k + 18k for a 350V HT (just a guess).

-Mark

Tim Shoppa wrote:
I have been playing around with my homebrew VFO, a Hartley oscillator,
with a tapped inductor in the tank.

Rough parameters: 6AH6 pentode for the VFO, tank resonant at 1.8Mc,
plate circuit resonant at 3.6Mc. Rather low-Q plate tank (on purpose, I
want it to cover 100kc or so). Tank tapped about 1/3 of the way from
the bottom. 150V from an 0A2 on the screen, bypassed by a 0.005 uF
ceramic with short leads at the screen.

While playing around with it I found this weird mode where it wouldn't
necessarily start up in constant oscillation. It would repeatedly
(30000 times a second) start up (starting up very quickly, in just a
few cycles), grid and cathode circuit amplitude would build up to about
10V p-p, then the oscillations would slowly (over the next 30
microseconds) die down. Then it would repeat.

Hand capacitance near the grid or on the tank coil would often break it
out of this mode and into more regular oscillation. Putting a 10x scope
probe on the grid sometimes broke it out of this mode too.

What eventually made the circuit more reliable was putting a few
hundred ohms in series with the grid. But I don't understand exactly
how this helped.

My guess for this squegging mode is that the oscillator would suddenly
start, the tank would ring, the tank would ring hard enough that grid
current flowed, and that the grid current somehow would "latch" on
until oscillation died out, then it would repeat.

Or, just maybe, it's more closely related to screen current and maybe
some kind of oscillation of the 0A2 in the screen regulator. I tried
changing the dropping resistor feeding the 0A2 and a couple different
0A2's but the squegging seemed relatively insensitive to it.

Trying some other pentodes that were not so "hot" (e.g. 6AU6, 6BA6)
changed the squegging a little but it was still there.

My not-understood fix, putting a few hundred ohms in series with the
grid, is something that I've often seen in real-world circuits. I think
this is to provide some degeneration, and always was under the
impression that the degeneration was intended to prevent oscillation in
the VHF range. It is possible that the circuit was indeed oscillating
at a few hundred Mc but my scope (100Mc bandwidth) didn't see it. And I
don't know how a 200Mc parasitic might cause 30kc squegging.

Any thoughts?

Tim KA0BTD