[Amps] PI-L Thoughts!

Ulf (SM0NOR) ulf at sm0nor.com
Sat Dec 11 10:26:16 PST 2010

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.


Ulf (SM0NOR)
ulf at sm0nor.com

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