>
>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.
>
>I built several varieties using both resistance wire and bus wire with
>varying turns and construction techniques. Thanks to Al Harbach who lives
>right down the road for access to the nichrome wire and the resistors.
>
>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.
>Initial attempts to make the measurement were unsuccessful because I tried
>hooking the network to the analyzer with clip leads and BNC adapters. The
>wire and the connectors added additional effects that cluttered up the
>results. I ended up using a test board that we keep on hand that has SMA
>connectors properly terminated to 50 ohm transmission line. I soldered the
>network in question to the board and then connected the test card to the
>analyzer.
>
>The nichrome behaved as advertised and lowered the Q of the resonance by a
>factor of about 10.
The important measurement is Q at c. 100MHz.
> 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.
>
>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.
>
>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.
>
>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. 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.
>
I doubt that a harmonic of the exciter could be a factor. IMO,
intermittent VHF parasitic oscillations principally take place in the VHF
resonant circuit between the anode and the tune capacitor. Whenever a
change in anode current takes place, damped wave ringing takes place in
the anode ressonant circuit. Some of this energy gets fed back to the
input due to feedback capacitance.
>The fact that many amplifiers work fine with the stock suppressors or even
>no suppressors doesn't surprise me.
The only currently manufactured ham amp with no suppressor whatsoever is
the AL-1500. I know of three that ate more than five 8877s.
> 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.
Some tubes seem to have more gain than others, esp. at VHF.
>
>Adding the resistance wire was painless and inexpensive. It seems like
>cheap insurance to me.
>
Resistance wire lowers VHF Q -- and that reduces the peak amplitude of
the VHF damped-wave ringing. . Glitch protection resistors and diodes
are the insurance.
cheers, Steve
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