Hmmmmnnnn- This all got my curiosity up enough to grab my reactance rule and
see what is going on .
If we use a 200 uhy choke at 160 meters - 1.8 mhz - its reactance is 2262
ohms, A capacitor that parallel resonates this at 1.8 mhz is 39 pfd so your
tune cap in a PI net would have to be 39 pfd larger than a "perfect -
invisible choke" - not very much compared to a 500 pfd or so tune cap.
Now if we pick a Q of 100 for the choke , Rp will be 226200 ohms (Rp=QXp) so
for a RMS plate swing of say 2000 volts the power lost in the choke will be
17.7 watts. (2000 squared/226200). The RMS RF current in the choke will be
8.8 ma.
A bypass capacitor at the "cold" end of the choke of 5000 pfd - reactance
would be 17.7 ohms or a voltage divider of about 226218/18 or 12568 times .
So for a 2000 volt RMS swing the 1.8 mhz voltage across the 5000 pfd bypass
would be 2000/12568 or about 160 millivolts.
If Q was only 50 then it would be 35 watts lost and probably would go up in
smoke and even the 17.7 watts is gonna be fairly warm if wound on teflon
rather than a ceramic rod.
So dunno off hand why the extra choke should have made such a significant
difference on 160 unless perhaps its Q were quite low , but then I would
expect it to go up in smoke anyhow. But what works is what is important !
73 de hank K7HP
> Many thanks, Gary & Paul, for your very informative feedback to my
> original head-scratching here, Hi Hi.
>
> Well, maybe I'm deluding myself here, but I like to at least *think* that
> I'm somewhat of a purist, too --- and when it comes to 160-meters, any
> residual effect(s) from less-than-stellar design / operating procedure of
> "linear" amplifiers can have unfortunate effects upon others using the
> band...and believe me, I KNOW what I'm talking about from first-hand
> experience here (I'm practise the old-school "...earn-as-you-learn"
> techniques, Hi).
>
> To that end, I think what I'm going to consider doing here (once the
> "regular" 160-meter season ebbs with QRN) is install TWO series chokes for
> my 813's, i.e. the big multi-pi National jobbie that's in there now,
> physically mounted below my 250-uh. solenoid choke: the tricky part will
> be devising some sort of a switch that would short-out the multi-pi choke
> for those bands other than 160.
>
> (BTW Gary, you are right on the money re. series resonances: my 250-uh.
> solenoid coil shows a dip at around 24-MHz, just as your 200-uh. coil
> did.)
>
> I think that a switchable choke arrangement is a FAR better option to
> consider, rather than building a separate, dedicated low-band
> amplifier...although if I could ever get my paws on a good 833A, or two, I
> just might eat those words! Hi Hi.
>
> The 250-uh. choke was the one that I originally used in the amplifier, and
> I 'd gallop merrily along literally all over the authorized SW spectrum
> with it, with narry a problem --- except on 160, where the amplifier just
> never quite seemed to tune-up properly (until I subbed-in the multi-pi
> National, that is).
>
> The more I know, there more there is to learn, it seems --- but one thing
> is for certain: Ham radio, as in life itself, is FULL of compromises!
>
> ~73~ Eddy VE3CUI - VE3XZ
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