Cc and C1 is what the two variable caps are labelled in the original
circuit, there's no special meaning beyond that. Cc is the series
coupling from the output, C1 the shunt capacitor.
I'd say it's misleading to call them 'tune' and 'load'. In a typical hf
pi network the load cap predominantly sets the wanted load resistance
and the tune cap adjusts the load to be purely resistive and (to a very
loose first approximation) each does their job independently of the
other. In the amp you're working on both caps affect both functions.
Think of the wanted load at the anode as a point on graph paper. In a pi
network 'load' cap moves your position left and right, 'tune' cap moves
you up and down. In your amp, one cap moves you on a slant / and the
other on a slant \ so it's a much more iterative process to find the
best tuning spot.
The original amp gives figures for running at 2.5kV and 1.2A - that's a
much lower load at the GS35 than you would target for 3.3kV and 0.8A.
Don't forget that connections around the blocking caps contribute to the
total series L in the circuit. Can you can patch in a short leaded
resistor of the desired load from anode to ground and look into the
output with a VNA to help preset the tuning on the amp when cold?
Good luck with the experiments, play safely.
73, Steve G8GSQ
OK Steve. Thanks for your valuable input.
Firstly, the loading adjust is very simple. Both the tune and load capacitors are simply
shhets of copper. The tune plate is wound in and out by a threaded rod. The load plate is
fixed at a 1/2" spacing. John added another threaded rod to enable that plate to be
to be pushed in, to give some adjustment capability. As it happens, we actually ended up
bending that plate further away, to increase the gap and loading and then brought it back
in to the best position with the threaded rod. So, we didn't actually change anything.
The loading plate is now probably effectively 1/8" further away.
I am hoping to get to John's place today or tomorrow, so I'll bring my laser
dot temperature thingy and we'll see what gives.
He did remove the plate bocking cap yesterday and showed me the picture. It has
sustained damage and I suspect it is toast. It could be our problem. It was
2200pF at 10kV. RF rating unknown but obviously not good.
I'm bringing 2 x 68pF 10kV 7kVAr doorknob caps and we'll put them in and hope
for the best.
I'm sorry but I'm not familiar with the meanings or C1, Cc etc but I guess they
may be Tune and Loading in my language. John is an engineer. He'll probably
slap me for not knowing. Hi hi.
Anyway, we have some more investigating and experimentation to do. I'll report
back with any successes oor continuing failures.
73, Alek, VK6APK
On 2/10/2022 9:12 pm, Steve wrote:
You mention adding 'the ability to adjust output loading'. What was added
and where? The original circuit does allow the loading to be altered, just not
in the way that you might be used to with 'tune' and 'load' controls on a pi
network.
Different combinations of Cc and C1 allow the load presented to the GS35 to
be made resistive and varied over a range but the tuning is very sensitive to
either of them being varied. I looked at it on a Smith chart to get a feel of
how it operates; I'm sorry but I lack the skill to explain it simply in any
other terms. Perhaps others can help.
Getting the wanted load at the anode is also quite sensitive to the
inductor and total capacitance to ground at the GS35 (which can be very much
more than the Cag figure in the data). Copying the circuit without knowing the
original layout in detail could easily throw the end result out.
From what you describe, it seems like a thermal thing; maybe valve
capacitance, maybe the inductor or capacitor(s) are heating up. All of them
could result in what you describe. Can you get an infrared camera to peer in
there?
Both anode and grid current high suggests to me that the load impedance is
too low. Less Cc and more C1 is the combination to push it higher.
I took a guess at component values to get a realistic end result and the
loaded Q at the L/C1/Cc junction can head up into the 30+ region. Increasing Cc
and C1, decreasing L results in lower loaded Q for the same load resistance at
the GS35, which might help, but you'll have to cut and try until it works.
73, Steve G8GSQ
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