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.