Rich writes:
>>there's a bit more to designing a VHF-suppressor than Q.
Absolutely true. The actual reactances of the pi or pi-L network are
critical, and one absolutely cannot intelligently design the suppressor
without first measuring those reactances, as they are critical to
knowing what the best suppressor is. Q of the suppressor almost doesn't
work into the equations at all.
>>It seems to me that an optimal suppressor design divides the VHF
>>signal
>>fairly equally between suppressor R (Rs) and the suppressor L .
This means the phase shift through the suppressor is 45 degrees. Why is
this optimal? Since our concern is what load impedance the anode sees,
would it not be more sensible to make the overall VHF current flow from
the plate to the entire output network 45 degrees, assuming this 45
degrees is an optimum of some sort?
Thus,
>>if the reactance of Ls is say100-ohms at the VHF anode-resonance, a
1-ohm
>>suppressor R (Rs) is not going to divide current as equally as a
100-ohm
>>Rs.
>>>If the suppressor has a reasonable path to ground at the matching
>>>circuit end, then it appears in parallel with whatever the valve
looks
>>>like from anode to ground.
>>>
>> .A large if. The sticky wicket is undoubtedly VHF resonances in
the
>>Tune-C that are in the vicinity of the anode-resonance.
Precisely. This is why the supressor pales to insignificance in most
designs. If my matching network has some sort of high impedance
resonance (which you allude to by claiming that a low impedance is a big
"if"), and since this network, at the intended frequency is several
kilohms, then at VHF must be several kilohms plus several more, then
what difference would putting a suppressor in series with it of a
hundred ohms or so make? The only time I've ever put a suppressor in an
anode is when my output network did look like a shunt to ground at VHF.
If it looked high impedance, then I found suppressors totally unneeded.
And in most cases, I do measure such high impedances, and use no
supressors. However, the ham amps, because your tuning capacitors are
physically small compared to what the commercial rigs use, probably do
actually have fairly low Z to ground and you do need the suppressors.
>>. So why not make Rs = 1-ohm in order to reduce the VHF load R on the
>>anode, and thereby reduce VHF gain?
Ah, no...a strangeness in terminology leads to this confusion.
"Reducing load" actually means raising the impedance, which raises gain,
and does not reduce gain.
There appear to be conflicting observations about a lot of things here.
Let me put it to the group. If anybody is interested in having this
subject put to a real test - not an argument, but a genuine experiement,
derived through proper DOE (Design Of Experiement) procedures, would the
interested parties ante up? I've an acquaintence who's very good at
DOE, and I'd be willing to un-modify my SB-220 (which I haven't used in
the ten years since my license expired, so the tubes are old) and donate
it to the cause. The participants who have theories to prove would have
to state the theories in a methodical manner, i.e.: First, this happens,
then this happens, and then that happens. The DOE experimenter would
outfit the SB-220 with appropriate measurement apparatus to catch these
brief events when they happen. There seems to be a lot of emotion in
this debate, and I've not seen anything in it that suggests that the
experiement would take nearly as much effort as people have put into
writing vitriol. If this is to work, cogent arguments must be put
forth, and more than one.
Arlen
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