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[AMPS] L-Pi net...downside?

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Subject: [AMPS] L-Pi net...downside?
From: measures@vc.net (Rich Measures)
Date: Sun, 1 Aug 1999 12:17:50 -0700


>
>OK, another question.  I'm designing the output network for my amp, and I'm
>facing the issue of getting a reasonable Q on 10 meters with practical 
>parts. 
>The 1999 ARRL handbook has a discussion of this, and they mention putting a
>small inductance in series with the plate of the tube, before the pi-net 
>input
>capacitor.  This has the effect of transforming the plate load impedance to a
>much lower value (in their example, from 2200 to 1200 ohms), so that the 
>pi-net can be designed with a more practical value of input capacity on the 
higher
>bands.  The effect of this inductance decreases with frequency, of course, so
>you can ignore it on the lower bands.  Sort of a "L-Pi" (or L-Pi-L) network!
>
>Recently I was looking at a 30S-1, and I noticed that the plate tuning 
>capacitor was a big air variable, right next to the shielding.  "Collins 
should know
>better", I thought, until I saw the little coil in the anode compartment!
>
>I know this was discussed a few months or a year ago, and I recall that KE9NA
>used this in his amp.  My question is:  what is the downside of doing 
>this?  It
>looks too much like a free lunch to me.
>
?  Murphy said that things are more complicated than they look.  In my 
opinion, this is prime example of what he was talking about.  



Rich...

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


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