[AMPS] Another Stupid Question

2 2@vc.net
Fri, 7 Dec 2001 07:57:59 -0800


>
>
>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!  :-)
> 
//  Marv --  Do the math and you will likely stow your wrath. 

Tom and I see pretty much eye-to-eye on specious Ham radio 
techno-traditions. 

cheers
>> 
>> 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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-  R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.  
end


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