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
>
>
>
> _______________________________________________ Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|