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!
Sent from my iPad
On 11 dec 2010, at 20:04, Vic K2VCO <vic@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@sm0nor.com
>>
>>
>>
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>
> --
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco/
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