To: <amps@contesting.com>
>> Date: Tue, 12 May 1998 19:34:17 -0500
>
> Hi Jon,
>.........
>> If the output Pi-network of the SB-220 has a low-pass response as most
>> output networks do, then what difference at VHF/UHF does the antenna make
>> anyhow?
>
>Absolutely no difference at all for VHF Jon. The tuning cap
>looks like a very low impedance, the tank inductor and leads a
>high series impedance, and the loading cap a virtual dead short.
>
>Been there, measured that.
>
I agree on the tank inductor point, Mr. Rauch. However, as I recall you
measured numerous VHF resonances in someTune caps., which means that
C-Tune is not necessarily a "very low impedance".
>The load, however, can upset the cart at HF near and below the
>operating frequency and lower all the way to VLF.
>
At VLF, a series1000pF HV-DC blocking capacitor is a high Z. At VLF, a
50uH shunt HV-RFC is a low Z. It looks to me like VLF would be virtually
blocked from the HF tank.
>It's a VERY simple job to break out a VHF signal source, drive the
>output port backwards, and measure RF voltages through the tank.
>
>It's only a tad more complex to add a few thousand ohm resistance,
>and drive the anode terminal of the cold tube while measuring
>voltages in the tank. It would take less time to do this than has
>been spent on soapboxs postulating all sorts of nonsense about
>extreme VHF voltages causing arcs, and VHF oscillations being load
>impedance sensitive.
Nonsense? Was it nonsense that caused this AL-82 bandswitch to arc for
no apparent reason?:
http://www.vcnet.com/measures/AL82.tbs.JPEG.
The bandswitch is interesting to me because the arc was so hot that it
caused the (alumina) ceramic to heat so rapidly that it fractured, and,
it caused shiny spots on the ceramic surface due to melting. (switch
courtesy of Randy Boyle, KT2D)
>
>I always make this type of measurement (using a sweep generator and
>RF detector in a network analyzer) in the normal course of
>prototyping new PA's. The voltage (transmission loss) smoothly and
>rapidly decreases as the probe is moved through the anode system
>towards the tuning cap. By the time the loading cap is reached,
>nothing measurable is left.
>
>The anode would have to swing tens of kilovolts at VHF and drive the
>system with many amperes of current at the same time to make the ten
>meter tap on the bandswitch arc at VHF.
The measured breakdown potential in a typical SB-220, TL-922, AL-80,
AL-82, on and on, bandswitch is about 6000v.
>
>It's bad enough science to claim a bandswitch or tuning capacitor can
>arc from a VHF parasitic far removed from the operating frequency, and
>worse yet to claim the VHF impedance at the output port has anything
>to do with VHF instability.
>
>For goodness sakes, common sense tells us that if the VHF energy
>could freely pass through the tank, the amplifier would be a harmonic
>disaster. It would never pass any sort of harmonic level testing.
>
sounds reasonable
>>Do we really need some sort of a load? After, everyone talks
>> about how the output Pi-Net works as a filter to remove the harmonics, so
>> if it removes (or reflects - The S11 of the filter at the frequency of
>> the harmonic energy is high) harmonics, won't it also reflect VHF/UHF
>> energy?
>
>It certainly does.
>
...unless the Tune-C has a resonance near the anode-circuit's VHF
resonance.
>..........
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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