[AMPS] Another Stupid Question

[2]

>
>>    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.
>
>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.
>
>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:
>
>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.
>
>As I said, I learned my lesson many years ago when I laughed at 
>someone for reading a blocking cap wrong by a factor of ten in a 
>160 meter amp. After poking fun at him, I was amazed when we 
>changed the capacitor to a "traditional" value of 1000pF and 
>nothing we could see changed.
>
>People waste far too much time worrying about enough blocking 
>capacitance, and spend far too little time considering current 
>ratings of the capacitors and the plate choke design.....which has a 
>MUCH larger effect than the blocking capacitor on system Q and 
>other problems.    
>
>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.  
>   
//  Score one slam dunk for Mr. Rauch.

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


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