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 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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