Hi Steve,
> It just so happens that I was building new suppressors for a Clipperton
> that I am returning to service and the parasitic topic comes up again.
> Since I have access to some very nice equipment, I can build the
> suppressors and measure them very accurately. I thought I would pass
> along my findings.
Can you measure S21, and have you measured it?
> My final network has eight turns of nichrome about .25 inches in diameter
> along side of a Matsushita 100 ohm 3 watt resistor. The trace reveals a
> very nice characteristic with a self resonance of 1.5 Gigahertz.
What is a "nice characteristic"? What was the goal?
> The nichrome behaved as advertised and lowered the Q of the resonance by a
> factor of about 10. At these very high frequencies, your results may vary
> so pay attention to the setup. Attempts to duplicate the resistance wire
> with an additional series resistance did not work as the body of that
> device added additional resonance albeit also at very high frequencies.
What frequency did you determine was involved in the problem you
saw? How did you determine the frequency of the problem?
> The construction technique was also critical. Wrapping the wire around
> the body of the resistance added capacitance lowering the resonance
> frequency. Winding spacing also changed the resonance.
What frequency are you talking about?
> Differences between the two types of wire were only evident at very high
> frequencies where it lowered the Q of the self resonance and very low
> frequencies where the network behaved as a pure resistor.
1.5 GHz is ultra high frequency. Why are you concerned about the
system at that frequency?
> With the network installed in the amplifier, one could identify a loop
> that included the plate capacitor, the capacitance of the plate choke, the
> high voltage bypass capacitor and the suppressor that could resonate at
> VHF frequencies.
What was the impedance these "problems" presented right at the
anode of the tubes? How much did you change the anode
impedance with you mods, as measured right at the anode?
Why is resonance bad? Can't it be good also?
Because of differences in construction, this resonance
> frequency may and will vary from one amplifier to another. If you happen
> to get one that resonates at a harmonic of the exciter frequency or at a
> frequency where there is sufficient gain from the tube, a parasitic may
> ensue.
What do harmonics have to do with parasitics?
If any oscillator has more regeneration than loss, it will oscillate. If
it doesn't, it won't. External excitation has NOTHING to do with
this, since even random shot or thermal noise in the circuit will
start the oscillation.
You have an oscillator, or you don't.
Now certainly gain has something to do with a parasitic, but it also
requires feedback of the correct phase and the correct impedances
at the proper spots are critical to this.
What did you do to define the problem? That would be valuable in
understanding if the cure was logical...or just an exercise in looking
at one narrow parameter that has little to do with the overall
problem.
> The fact that many amplifiers work fine with the stock suppressors or even
> no suppressors doesn't surprise me. The variation in construction may
> move the resonance to frequencies where there is insufficient gain to
> cause a problem. This would make some systems more likely to fail than
> others.
I can make any stock Clipperton, or any Clipperton blessed with
nichrome, oscillate by misterminating the input and/or output, and
adjusting the controls "properly" provided nothing is done to
address the HF feedthrough problems.
73, Tom W8JI
w8ji@contesting.com
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