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

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.
(snip)

(my guess)
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. Adding the series
resistor reduced the efficiency of the rectification.

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

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.

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

Spice is your friend. I have found that LT spice is particularly good
at predicting squegging, at least in general. There's never a 1:1
correspondence with the real thing, but close is pretty good in RF.

When I have experienced squegging like this it has been because I have
built a circuit with a resonance between the circuit capacitances and
the RF chokes (you know -- those things you use for biasing that you
treat as 'shorts' in your AC analysis?). To the bias circuit, the
oscillator looks like a negative resistance, so you get two modes of
oscillation at once.

This is complicated by the fact that the circuit wouldn't oscillate at
the lower frequency at all if it weren't for the action of the intended
oscillation -- it's the current requirements of the active device as
oscillation build up that cause the negative resistance action that
causes squegging.

I suspect that your grid resistor is killing the Q of the circuit at
30kHz, which kills the squegging without killing the intended mode of
oscillation. You may find that loading any bias chokes with carefully
selected, unbypassed series resistance will also kill the squegging.
You can also sometimes kill it by reducing selected bypass capacitances
to lower the circuit Q at the squegging frequency.

Of course all of this also lowers the circuit Q at the desired
oscillation frequency -- that's life. Your job is to find a happy
medium that gives you good oscillator performance without squegging. Or
start building superregenerative receivers!

--

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

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 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.

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.