[AMPS] Re: Solenoid Chokes

[Roy Koeppe]

A question I have is : Is there a problem with too much of a good thing?
I wind my chokes so that no resonances fall within a desired operating
frequency. The belief is that it contributes to stability by not having
another potential tuned circuit to deal with.
Guess I'm of the old school that remembers putting a resistor across a
choke to de- Q it without really examining the reason.
I have noted that in a 6M amp, when I place the series resonance around
70MHz the amp is extremely easy to tame. That is not the case when the
resonance is around 50MHz.  Something to do with poor input to output
isolation in a GG amp ??

Hi Carl; you wrote the above (and I'm sure you meant to say PARALLEL
resonances in the above),

Well, when a choke is resonant ABOVE the operating frequency, I too notice
things go easier. I'm used to that condition on 160M, where a
self-parallel-resonant choke, an inch in diameter requires about 350 turns!
So in this case I went  for a parallel resonance closer to 80M or even
above. (For the reasons you listed in an earlier post--difficulties on the
higher bands with many turns). With the resonance above F-Op, the choke
reflects what I have heard called, "negative inductive reactance" into the
tank circuit. It has that familiar effect of canceling some of the capacity
of C1, forcing us to increase the value of C1 higher than predicted. More
circulating current also flows in the choke winding.

Now, when the choke's fundamental resonance is right on F-Op, no reactance
is reflected in, and the choke's Z is very high indeed. I'm doing this on
80M. But, the same choke on 40M reflects "negative capacitive reactance"
into the tank, causing C1 to be smaller than predicted, and again some
additional circulating current flows in the choke. All this tested by
dipping, and on the air. (This amp now covers only 80M at present, purist
style). Above 40M a series resonant mode happens, so this single-segment
choke is not for multi-band use at all.

About your instability situation, the type of reactance your tank "sees"
with the 50 mHz or the 70 mHz resonance inserted, must be influenced  in
much the same manner as if you placed a small shunt cap across the choke to
ground, or did not do so. The reactance of the higher-resonace choke must
counteract some other undesireable VHF resonance, maybe? That's all I can
come up with, but it's fun imagining    :-)

73,    Roy    K6XK




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