"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.