[Amps] PI-L Thoughts!

[Vic K2VCO]

There's one more possibility that you haven't mentioned: add a small inductance between 
the plate and the input of the pi-network. This makes it an "L-Pi" or "L-Pi-L" network. 
The effect is to transform the impedance to a lower value BEFORE the normal pi section so 
you can use more capacity and still have a decent Q. The inductor will have a large effect 
on the highest bands, but be insignificant on the lower bands.

This is the approach I've taken with my 2x813 amplifier, where the output capacity for 2 
tubes is 28 pf. -- and that's before the circuit strays, RF choke, etc.

On 12/11/2010 10:26 AM, Ulf (SM0NOR) wrote:
> 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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-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/