[Amps] PI-L Thoughts!

[Ulf (SM0NOR)]

Ok. Thanks. I have come across this solution before. I kind of didn't think about it, because it is not so common. I'll look into it!

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On 11 dec 2010, at 20:04, Vic K2VCO <vic at rakefet.com> wrote:

> 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/
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