On Sat, 13 Mar 1999 13:40:21 -0600 "Roy Koeppe" <royanjoy@ncn.net>
writes:
>
>Some general stuff about xmitting chokes: First, like with
>cholesterol,
>there are two kinds of self resonances--good and bad. The good
>resonance is
>the parallel and the bad is the series. In fact, for the highest
>perfomance
>possible, a single-band amp's plate choke should be naturally
>parallel
>resonant on that band. Parallel resonance is measured without a short
>applied across the choke. Series resonance is measured with the short.
>Both
>resonance modes are ideally measured with the choke "in place" in the
>PA
>compartment.
>
>Parallel resonance is analogous to using a tuned choke as the choke
>input
>filter in a power supply. The effective Z is much higher than the
>inductive
>reactance would be of the un-tuned choke alone.
>
>Impedances of solenoid chokes are very high because of natural
>parallel
>resonances. In fact, there are multiple parallel resonances because
>of the
>structure of the solenoid. The main, fundamental resonance in a
>practical
>choke can exhibit an EFFECTIVE parallel reactance of up to 1-megohm.
>Useful
>areas around the main peak commonly have effective reactances of
>200,000
>ohms. This is how solenoid chokes "work."
>These effective reactances are far, far in excess of simple
>inductance
>values given by manufacturers or by calculations for inductance, which
>are
>LOW FREQUENCY values only.
>
>To prove this without a doubt, calculate the inductive reactance only,
>using
>the simple choke inductance value as specified in typical amps. About
>1 mH?
>Now, for 1.8 mHz, that's around 12,000 ohms? Far from 200,000 or a
>million.
>And with say 4 kV swinging at the "top" that equals excessive
>circulating
>currents within the choke. Fortunately, that's not the case.
>
>It's the self-resonance (good kind) that is in service for us.
>
>Hit me.
Why? I think that was an excellent post Roy.
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 ??
73 Carl KM1H
73, K6XK
>
>
>
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>
>
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