[Amps] PI-L Thoughts!

[Jim W7RY]

Please look at the way I handled a low minimum value cap in my 3CX800 amp. I 
used a small cap for 10, 15 and 20 meters. Then switched in a larger cap for 
40, 80 and 160.

Pictures @

http://s900.photobucket.com/albums/ac206/w7ry/3CX800%20Amplifier/

73
Jim W7RY

--------------------------------------------------
From: "Ulf (SM0NOR)" <ulf at sm0nor.com>
Sent: Saturday, December 11, 2010 10:26 AM
To: <Amps at contesting.com>
Subject: [Amps] PI-L Thoughts!

> Hi all,
>
> I'm looking for some advice regarding the tank.
>
> I'm about 2/3 on the way to complete my first amplifier build and I'm 
> having some thoughts about how to get C1 to low enough to load on 10 
> meter.
>
> To no surprise the 25-300 pF capacitor measured a min of 40 pF when 
> mounted in the RF deck. This will give me trouble on the higher bands. I 
> know this is discussed a lot in the literature, but I'd like your thoughts 
> on one specific solution.
>
> The obvious solutions are to either get another C1 with a lower min 
> capacitance or to get a vacuum variable. Maybe I could do the first 
> solution, but that would probably make it necessary to rebuild the 
> switching section with another deck to be able to switch in different 
> sections of a split stator cap. The vacuum solution is really easy, but I 
> have not really budgeted for that.
>
> This makes me think of a third solution mentioned in the handbooks but not 
> really discussed in depth; and that is to tweak the Q to make it work. And 
> I would like to have the opinions of all you experienced builders out 
> there!
>
> Here are my calculations (based on one of the handbooks)
>
> C1 C2 L1 Q
>
> 160 330 1400 30.4 11
> 80 174 716 14.1 12
> 40 96 388 7.2 12
> 20 43 182 3.9 11
> 15 45 161 1.7 16
> 12 43 151 0.7 18
> 10 43 143 0.6 20
>
> For simplicity I did not add the values for L2 in this post.
>
> Ok, what do you think of this? Is it Ok with a Q of 20 on 10 meter? I'm 
> using 5 mm copper tubing in the 10-20 coil. Or would it be better with one 
> of the other solutions? Tweaking the Q is by far the most simple solution 
> given the components I already have.
>
> Thanks for any advice!
>
> By the way.....   the build is based on a GU74B and I'm aiming for a 1KW 
> PEP.
>
>
> 73's
>
> Ulf (SM0NOR)
> ulf at sm0nor.com
>
>
>
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