[AMPS] Cathode input networks resonant or not?

[Rich Measures]

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>-----Original Message-----
>From:	Rich Measures [SMTP:measures@vc.net]
>Sent:	Saturday, August 29, 1998 5:32 AM
>To:	'amps@contesting.com'
>Subject:	Re: [AMPS] Cathode input networks resonant or not?
>
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>>>I submit - admittedly without PhD-level academic documentation but based 
>on
>>>hands-on experience - that if the cathode network isn't resonant AT OR 
>>>FAIRLY NEAR the operating frequency there's no "flywheel."
>
>>>......
>>the hands-on measured frequency of resonance of a pi-network (2
>>pi-networks in series) 

(should have read two L-networks in series)

>>is near the operating frequency, however, the two
>>frequencies are not the same.
>
>>Rich...

>Presumably you "hands-on" MEASURED the resonant frequency with the amp 
>turned OFF, or at least with tubes biased to cutoff. In either case the 
>tubes look basically like a capacitance across the output of the input 
>pi-net.
>
>Presumably also, you'd ADJUSTED the pi-net WITH DRIVE so that the tubes 
>looked more or less like a capacitance (which probably varies a bit over 
>the cycle) in parallel with a highly nonlinear resistance.
>
>That (nonlinear) resistance is across only part of the total pi-net 
>capacitance which determines resonance. I think a simple experiment will 
>show that the "dip" frequency of a pi is somewhat dependent upon the 
>terminating load R. That's what I seem to recall theory predicts. Maybe 
>it's no mystery!
>
It may be more accurate to say that the wider the Zin/Zout ratio of a 
pi-network, the closer the near resonance dip is to the operating 
frequency.  In an 80m, Q=10, 2k-ohm to 50-ohm pi-net. tank, as I recall, 
the observed dip was several hundred kHz below the operating frequency.  



Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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