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[AMPS] Input circuit

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Subject: [AMPS] Input circuit
From: (Jon Ogden)
Date: Fri, 06 Jul 2001 07:39:26 -0500
on 7/6/01 5:16 AM, Peter at wrote:

> Is there any source for a input circuit for a 4-1000 @ the hf bands? A
> layout of a print circuit with components is also welcome.

Hi Peter,

I think most people sort of use a "dead bug" type configuration for their
input circuits.  At least that's what I used.  It was easier that way to
group all the proper values around my input bandswitch.  Keeps lead lengths
shorter, etc.

Here's some tips to get you started:

Driving impedance of a 4-1000A in grounded grid is about 105 Ohms.
Input capacitance is about 23 pF if I remember correctly (in datasheets).

Here is the tip that I used to approximate things:

Pi-Network:  C2 = capacitor closest to the source
             C1 = capacitor closest to tube

Set C1 to approximately 13 pF per meter of wavelength.
Set C2 to be approximately 1.5 times C1.

Figure your inductor value appropriately from this.

Remember, that you will need to absorb the value of the input C of the tube
into your value of C1.  So for example, if you calculate C1 to be 250 pF,
then the actual component value you would use would be 250 - 23 = 227 pF.

The circuit Q of the pi-net should be around 2 to 3.

I found that the rule of thumb given above got me in the neighborhood, but
not completely there.  I had to do some trial and error to get the values
properly figured out.  I used fixed Cs and and adjustable L.  If I had to do
it over again, I think adjustable Cs and a fixed L would be easier!

Your mileage may vary also depending on layout, etc.  Try to get the input
pi-net as physically close to the tube as possible.

I can send you some of the values I used later if you want, it may or may
not help.  I am late for getting ready for work now and don't have time to
look them up!

Have fun!



Jon Ogden
NA9D (ex: KE9NA)


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