>
>On Sun, 2 Dec 2001 18:44:12 -0500 "Tom Rauch" <w8ji@akorn.net> writes:
>> > At higher frequencies, where given a typical tube the plate C
>> > represents a significant part of the plate tune cap, the effect is
>to
>> > significantly raise the plate impedance (as seen by the Pi-Net input)
>> > and the balance of the pi-net would need to be adjusted to maintain
>> > the desired Q & impedance ratio.
>>
>> Not true. It sounds "nice", but it is not factual. In truth, the
>> impedance barely changes.
>
> Oh, bah humbug. I'm disappointed Tom. Not only have you adopted
>Rich's style of changing the parameters of the discussion
// The Red Herring Card.
> to acheive your
>desired results but, you have his acquiescence! :-)
>
>>
>> Let's assume the blocking cap, at the lowest frequency, would be
>> set a reasonable value of 10% of the impedance looking into the
>> tank, or approximately equal to the value of the tuning cap with a Q
>> of ten (using the simple but not absolutely correct value of Rp/Xc.
>
> You have inserted the phrase: "at the lowest frequency" which was not
>a feature of the original question. The "10 % of the impedance" was
>formerly at the frequency of operation.
>
>> Using an Xc equal to 10% of the plate operating impedance, and
>> assuming a choke somewhat larger than the plate impedance, the
>> change in impedance looking into the tank is:
>
> Wrong! The plate choke may not be ignored as it is typically only
>"marginally larger" than the plate impedance in 160 meter ham amps. I
>stated in my previous message that it is a worse offender, at this point,
>than the coupling cap in the real world.
>
>> 300pF blocking (~300ohms 1.8 MHz) 2878ohms
>>
>> 1200pF blocking (~75 ohms 1.8MHz) 2964ohms
>>
>> The tank input impedance, with no other changes except a
>> readjustment of the tuning cap by 2 pF to compensate for the
>> reactance change, changes less than 100 ohms out of 3000
>> ohms....a totally insignificant change.
>>
>> And the small effect above is on 160 meters, where the problem is
>> at its worse point. On higher bands, the effect would be less
>> because Xc would decrease with increasing frequency.
>
> Today it does, but, look at the way the question was originally posed,
>where for instance 50pf could be used on 20 meters, then the 10pF
>"typical" tube output C comes into play and you'll see what's troubling.
>
> Now, at the risk of prolinging this thread, I'll just add the comment
>that the smaller value (<500pF) ceramic doorknobs are typically
>fabricated from lower loss material than their larger valued cousins.
>If one chooses to use a lower valued part, thus making it part of the
>matching network calculations, these small guys are better suited to the
>task.
>
>73 & Good morning,
> Marv WC6W
>
>
>
>
>
>
>*
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>--
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>
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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