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?

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.

Tim.

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.

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.

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

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

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.

"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.

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

What you write, which relates the Q (probably very low hundreds) of the
tuned circuit and it's frequency to the period the squegging, is in
good agreement with the squegging I observe. Will check the DC grid
voltage with the scope tonight.

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 grid tuned circuit is at 1.8Mc, and the squegging seemed
insensitive to the coupling cap. 100pF is the handbook value, but I did
play around with it. Too small (39pF) and no oscillation. At 56pF there
was squegging.

Did not try playing with the grid return value (still at 47K from the
handbook), that may indeed be the key.

Tim.