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.

Any thoughts?

Tim KA0BTD


I normally don't post, but, since I have done my own amount of torturing of
the 6AH6, I think i know what the problem is.

The 6AH6 is slightly misnamed.
The 6AH6 isn't really a pentode.
It's just a really small beam power tetrode.
It has usable gain up to the UHF region.
And it suffers a lot of the same personality disorders of it's larger
brothers.

It's rated at 3.2 watts plate dissipation.
But I have ran it close up to about 12.5 watts for extended periods.
At that point, the tube starts to show obvious signs that it's close to the
breaking point.
The plate starts glow red, but it didn't kill it.
Probably hurt it a good amount, but it didn't kill it.

The more current it pulls, and the lower the input and output impedance,
the higher the tendency for it to break into oscillation.
It will oscillate up to the UHF region easily.
It's preferred method of oscillation is the grounded grid form.
The cathode is the input, and the anode is the output.

When you have low resistance feedback loop from the plate to the cathode
over six or so inches long, and start pulling a good bit of power, then it
can go at any time.

Look up parasitic oscillation in your handbook..
You will see the circuit that is causing it.
Large power tubes top out at 100Mc or so.
But this small one can go up to 400 to 900Mc if it's being driven hard
enough.

The loop is most likely from your plate, through the tuning cap of the
output tank.
The cap will look like a short circuit at UHF.
It will then go through the chassis.
And then through the cathode tank, which is shorted by the tank cap at UHF.

Ways to solve the problem.
Turn down the plate, and screen voltage.
It will drop beam current.
That will increase the resistance of the tube to the point it can't
parasitically oscillate.

Or you could put a 100 ohm or 1000 ohm resistor in the plate circuit.
In big amplifiers, where you can't tolerate a large resistance in the plate
lead,
they use parasitic chokes.
The appear as a short to DC, and the desired operating frequency.
But they appear as a high resistance to the VHF oscillating frequency.
It basically de-Qs the VHF oscillating circuit.

But you can tolerate a little bit of loss in the plate circuit of this unit.
So, just put a 100 ohm resistor in series with the plate.
If that don't work, then try a 1K resistor.
I am pretty sure that it will settle down and behave.
It will be a lot easier that winding a parasitic choke for it.

You also want to take a careful look at the screen circuit.
It can also act like an output in a parasitic oscillation.
So, try putting a 100, or 1K resistor in series with it too.

It's oscillating cycle.
It starts it's HF oscillation gain cycle.
The signal grows, and the tube bias increases.
The current on the positive peaks grown with it.
The current on the positive peaks get to the point that
the tube breaks into UHF oscillation.
The UHF oscillations are several times larger than the intended oscillation.
The UHF oscillation only last a fraction of the high frequency cycle.
When it starts it's UHF oscillation, it quickly charges the grid capacitor
up to a very high level.
Way past cutoff.
The UHF oscillation quickly extinguishes it's self.
But since the tube is in cutoff, the intended HF oscillation dies too.
The grid voltage drops back to normal, and the tube starts the cycle again.

Other ideas
You could also increase the grid leak resistance.
Push it up to 1M or so.
It will decrease the tank loading, and allow it to start easier.
And it won't pull so much positive peak grid current,
and cause the tube to conduct so heavily on the positive peaks.

You can tease out the UHF oscillation into the open if you short the primary
grid tank,
so it can't oscillate at it's intended frequency.
Basically, zero signal condition.
Then, push up the plate, and screen voltage.
If you have a little signal strength meter that works up to the UHF range,
with it's antenna clipped to the chassis, or wiring.
You will se it start to come alive.
You will be able to move your hand around the unit,
and it will break into, and out of oscillation.
Sometimes, it will motorboat, or operate continuously.

You want to change the circuit so that you can't get it go parasitic,
even when you really push it to it's limits..

Then un-short the tank and you are ready to go.

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

N9WOS wrote:
I normally don't post, but, since I have done my own amount of torturing of
the 6AH6, I think i know what the problem is.

The 6AH6 is slightly misnamed.
The 6AH6 isn't really a pentode.
It's just a really small beam power tetrode.
It has usable gain up to the UHF region.
And it suffers a lot of the same personality disorders of it's larger
brothers.
Large power tubes top out at 100Mc or so.
But this small one can go up to 400 to 900Mc if it's being driven hard
enough.

The loop is most likely from your plate, through the tuning cap of the
output tank.

OK, I saw the parasitic last night!

I only have a 100MHz scope. Putting a scope probe on the circuit seems
to often kill the squegging. But I put a little loop on the end of the
scope probe and sniffed around, and indeed there was a 400MHz parasitic
that would build up over maybe 0.1usec. Then the impulse from this set
the tank ringing at 1.8MHz, and after the ringing mostly decayed
(30usec or so) the cycle would repeat.

There seemed to be the most energy at the well-bypassed screen and not
at the plate, but remember this is an electron-coupled oscillator so
the screen is working as a plate.

I guess even a 0.001uF ceramic cap with short leads isn't a good bypass
for 400MHz!

There must've been substantial 400MHz energy to show up on my 100MHz
scope :-).

My band-aid of a resistor in series with the grid seems to do the
trick.

Tim.

Tim Shoppa wrote:
N9WOS wrote:

I normally don't post, but, since I have done my own amount of torturing of
the 6AH6, I think i know what the problem is.

The 6AH6 is slightly misnamed.
The 6AH6 isn't really a pentode.
It's just a really small beam power tetrode.
It has usable gain up to the UHF region.
And it suffers a lot of the same personality disorders of it's larger
brothers.
Large power tubes top out at 100Mc or so.
But this small one can go up to 400 to 900Mc if it's being driven hard
enough.

The loop is most likely from your plate, through the tuning cap of the
output tank.


OK, I saw the parasitic last night!

I only have a 100MHz scope. Putting a scope probe on the circuit seems
to often kill the squegging. But I put a little loop on the end of the
scope probe and sniffed around, and indeed there was a 400MHz parasitic
that would build up over maybe 0.1usec. Then the impulse from this set
the tank ringing at 1.8MHz, and after the ringing mostly decayed
(30usec or so) the cycle would repeat.

There seemed to be the most energy at the well-bypassed screen and not
at the plate, but remember this is an electron-coupled oscillator so
the screen is working as a plate.

I guess even a 0.001uF ceramic cap with short leads isn't a good bypass
for 400MHz!

There must've been substantial 400MHz energy to show up on my 100MHz
scope :-).

My band-aid of a resistor in series with the grid seems to do the
trick.

Tim.

Your resistor in series with the grid would have much more effect on a
UHF oscillation than on one at 30kHz, indeed.

Even a 1nF chip cap soldered directly between the screen lead and
cathode of the tube would have the internal tube lead inductances to
contend with, so you're probably never going to 'get there from here' by
that method.

I wonder if a loading resistor in the screen lead instead of the grid
would work?

--

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

N9WOS wrote:
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.


Any thoughts?

Tim KA0BTD



I normally don't post, but, since I have done my own amount of torturing of
the 6AH6, I think i know what the problem is.

The 6AH6 is slightly misnamed.
The 6AH6 isn't really a pentode.
It's just a really small beam power tetrode.
It has usable gain up to the UHF region.
And it suffers a lot of the same personality disorders of it's larger
brothers.

It's rated at 3.2 watts plate dissipation.
But I have ran it close up to about 12.5 watts for extended periods.
At that point, the tube starts to show obvious signs that it's close to the
breaking point.
The plate starts glow red, but it didn't kill it.
Probably hurt it a good amount, but it didn't kill it.

The more current it pulls, and the lower the input and output impedance,
the higher the tendency for it to break into oscillation.
It will oscillate up to the UHF region easily.
It's preferred method of oscillation is the grounded grid form.
The cathode is the input, and the anode is the output.

When you have low resistance feedback loop from the plate to the cathode
over six or so inches long, and start pulling a good bit of power, then it
can go at any time.

Look up parasitic oscillation in your handbook..
You will see the circuit that is causing it.
Large power tubes top out at 100Mc or so.
But this small one can go up to 400 to 900Mc if it's being driven hard
enough.

The loop is most likely from your plate, through the tuning cap of the
output tank.
The cap will look like a short circuit at UHF.
It will then go through the chassis.
And then through the cathode tank, which is shorted by the tank cap at UHF.

Ways to solve the problem.
Turn down the plate, and screen voltage.
It will drop beam current.
That will increase the resistance of the tube to the point it can't
parasitically oscillate.

Or you could put a 100 ohm or 1000 ohm resistor in the plate circuit.
In big amplifiers, where you can't tolerate a large resistance in the plate
lead,
they use parasitic chokes.
The appear as a short to DC, and the desired operating frequency.
But they appear as a high resistance to the VHF oscillating frequency.
It basically de-Qs the VHF oscillating circuit.

But you can tolerate a little bit of loss in the plate circuit of this unit.
So, just put a 100 ohm resistor in series with the plate.
If that don't work, then try a 1K resistor.
I am pretty sure that it will settle down and behave.
It will be a lot easier that winding a parasitic choke for it.

You also want to take a careful look at the screen circuit.
It can also act like an output in a parasitic oscillation.
So, try putting a 100, or 1K resistor in series with it too.

It's oscillating cycle.
It starts it's HF oscillation gain cycle.
The signal grows, and the tube bias increases.
The current on the positive peaks grown with it.
The current on the positive peaks get to the point that
the tube breaks into UHF oscillation.
The UHF oscillations are several times larger than the intended oscillation.
The UHF oscillation only last a fraction of the high frequency cycle.
When it starts it's UHF oscillation, it quickly charges the grid capacitor
up to a very high level.
Way past cutoff.
The UHF oscillation quickly extinguishes it's self.
But since the tube is in cutoff, the intended HF oscillation dies too.
The grid voltage drops back to normal, and the tube starts the cycle again.

Other ideas
You could also increase the grid leak resistance.
Push it up to 1M or so.
It will decrease the tank loading, and allow it to start easier.
And it won't pull so much positive peak grid current,
and cause the tube to conduct so heavily on the positive peaks.

You can tease out the UHF oscillation into the open if you short the primary
grid tank,
so it can't oscillate at it's intended frequency.
Basically, zero signal condition.
Then, push up the plate, and screen voltage.
If you have a little signal strength meter that works up to the UHF range,
with it's antenna clipped to the chassis, or wiring.
You will se it start to come alive.
You will be able to move your hand around the unit,
and it will break into, and out of oscillation.
Sometimes, it will motorboat, or operate continuously.

You want to change the circuit so that you can't get it go parasitic,
even when you really push it to it's limits..

Then un-short the tank and you are ready to go.




Nice post. A while ago, I went into "hair-pulling mode"
the other way - *trying* to cause squegging in a one
transistor DC-DC booster. Finally got it with a bit
of fiddling. Maybe it belongs under Murphy's section
on oscillators:

When you don't want oscillation, it will oscillate
When you want oscillation, it won't
When you want to get rid of squegging, you can't
When you want squegging, it won't

Do all Parisitics (sic) come from Paris? :-)

Ed

"Tim Wescott" wrote in message
...
Tim Shoppa wrote:
N9WOS wrote:

I normally don't post, but, since I have done my own amount of
torturing of
the 6AH6, I think i know what the problem is.

The 6AH6 is slightly misnamed.
The 6AH6 isn't really a pentode.
It's just a really small beam power tetrode.
It has usable gain up to the UHF region.
And it suffers a lot of the same personality disorders of it's larger
brothers.
Large power tubes top out at 100Mc or so.
But this small one can go up to 400 to 900Mc if it's being driven hard
enough.

The loop is most likely from your plate, through the tuning cap of the
output tank.


OK, I saw the parasitic last night!

I only have a 100MHz scope. Putting a scope probe on the circuit seems
to often kill the squegging. But I put a little loop on the end of the
scope probe and sniffed around, and indeed there was a 400MHz parasitic
that would build up over maybe 0.1usec. Then the impulse from this set
the tank ringing at 1.8MHz, and after the ringing mostly decayed
(30usec or so) the cycle would repeat.

There seemed to be the most energy at the well-bypassed screen and not
at the plate, but remember this is an electron-coupled oscillator so
the screen is working as a plate.

I guess even a 0.001uF ceramic cap with short leads isn't a good bypass
for 400MHz!

There must've been substantial 400MHz energy to show up on my 100MHz
scope :-).

My band-aid of a resistor in series with the grid seems to do the
trick.

Tim.

Your resistor in series with the grid would have much more effect on a
UHF oscillation than on one at 30kHz, indeed.

Even a 1nF chip cap soldered directly between the screen lead and
cathode of the tube would have the internal tube lead inductances to
contend with, so you're probably never going to 'get there from here' by
that method.

I wonder if a loading resistor in the screen lead instead of the grid
would work?

Yes.
My original reply also mentions that.

.....................................
You also want to take a careful look at the screen circuit.
It can also act like an output in a parasitic oscillation.
So, try putting a 100, or 1K resistor in series with it too.
.....................................

He says that the .001uf cap isn't much good at bypassing.
The problem is it's too good at bypassing.
It acts as a short to the UHF parasitic.
That allows enough UHF energy to be conducted to the chassis to allow it to
go parasitic.
Shorting the energy to the chassis, isn't the cure to the problem, its the
cause of the problem.

So I will clarify my previous statement.

You want a 100, or 1000 ohm resister in series with the screen.
Between the bypass cap, and the screen.
Not between the bypass cap, and the supply.

You want to isolate the capacitance from the screen.
That will break the loop required for oscillation.

When you don't want oscillation, it will oscillate
When you want oscillation, it won't
When you want to get rid of squegging, you can't
When you want squegging, it won't

I have a hard problem making a amplifier that can put out a solid 1.8Mhz
signal.
But I can build a DC power supply that produces a quality 80Mhz signal.

Maybe, if i want an oscillator, I should try to build a DC supply circuit,
and vice versa.
I will figure it out one of these days.

Do all Parisitics (sic) come from Paris? :-)

Ed

They always talk about the French whine. :-)

Since a 220 pf capacitor with ½" leads self-resonates somewhere around 150
MHz., you might want to do the old "put a variable capacitor between screen
and cathode and tune for minimum squeeg" trick. All you are doing is making
a series resonant circuit with the stray inductance of the tube and the
variable capacitor to short whatever nasties you don't want out.

Jim


"Tim Wescott" wrote in message
...
Tim Shoppa wrote:
N9WOS wrote:


Even a 1nF chip cap soldered directly between the screen lead and cathode
of the tube would have the internal tube lead inductances to contend with,
so you're probably never going to 'get there from here' by that method.

I wonder if a loading resistor in the screen lead instead of the grid
would work?

--

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

N9WOS wrote:
You also want to take a careful look at the screen circuit.
It can also act like an output in a parasitic oscillation.
So, try putting a 100, or 1K resistor in series with it too.

Indeed this is a working fix, as I verified on the bench.

It's oscillating cycle.
It starts it's HF oscillation gain cycle.

Actually in my particular case it appears that the UHF parasitic starts
first. But probably just the luck of the bias point.

The signal grows, and the tube bias increases.
The current on the positive peaks grown with it.
The current on the positive peaks get to the point that
the tube breaks into UHF oscillation.
The UHF oscillations are several times larger than the intended oscillation.

Very true here - this is in fact the only time I've seen 400MHz stuff
on my 100MHz scope, so I'm guessing that there is substantial UHF
energy there.

The UHF oscillation only last a fraction of the high frequency cycle.

This part is very true too. Tiny tiny fraction of a microsecond. My
interpretation of the scope trace is that the UHF activity gives the
the HF tank a "kick" and it rings for a while.

Tim.

On Thu, 04 Jan 2007 08:39:34 -0800, Tim Shoppa wrote:
N9WOS wrote:
You also want to take a careful look at the screen circuit. It can also
act like an output in a parasitic oscillation. So, try putting a 100, or
1K resistor in series with it too.

Indeed this is a working fix, as I verified on the bench.

It's oscillating cycle.
It starts it's HF oscillation gain cycle.

Actually in my particular case it appears that the UHF parasitic starts
first. But probably just the luck of the bias point.

Check for dynatron oscillations; the screen acts as a plate, sort of.
http://en.wikipedia.org/wiki/Dynatron_oscillator

I can't find a schematic of one, but it's just an oscillator with feedback
from the screen to the grid.

How long are the leads of your screen bypass cap, and where are they
soldered down?

Thanks,
Rich