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[AMPS] 4X1 tuned input

To: <amps@contesting.com>
Subject: [AMPS] 4X1 tuned input
From: jono@enteract.com (Jon Ogden)
Date: Mon, 12 Apr 1999 00:03:53 -0500
>In simple terms, can someone send me directions on how to make a tuned input 
>for my amp?

Here goes my version:

Bill Orr suggests (in one of his handbooks) using about 13 pf per meter 
wavelength for C2 (closest to tube).  On 80M this ends up being about 
1040 pF which is almost exactly what I used.  C1 is about 1.5 times C2.  
Then you need to adjust L accordingly for a match.  This works OK until 
you get to higher bands (15M and 10M) where other parasitic circuit 
elements start affecting things.  Also, you need to subtract the values 
of the tube's cathode capacitance from the value of C2.

Another good thing to do is figure that you need at least a Q of 2 for 
your input circuit.  Q is roughly Rin/Xc1 where Xc1 is the reactance of 
C1 in your pi-network.  Since Rin is 50 Ohms, for a Q of 2, Xc1 = 25 
Ohms.  

Rich suggests setting C1 for a reactance of 25 Ohms.  Then take a 
variable L and a variable Cap for C2.  Tune up both with the amp being 
driven at max power and tune for minimum SWR.  Then go back, measure L 
and C2 and replace with fixed values.  Kind of a pain, IMO, but it will 
give you very good results.

My suggestion is to simulate the input impedance of the tube with a 
couple of resistors.  Normal Radio Scrap types work fine.  Take a 220 Ohm 
resistor from each cathode pin to ground (Driving impedance of the 4-1K 
is 110 Ohms).  Put the tube in the socket.  Using an MFJ-259 or similar, 
tweak the values of L and C2 until you get your good match (set C1 for a 
reactance of 25 Ohms).  This method will get you quite close.  The only 
band I am above 1.2:1 on is 10M where I am 1.4:1.

Lots of guys say to use torroids for your inductors as slugs can 
saturate.  I used slug tuned coils since that is what I had and what my 
mechanics were set up for.  If you want more exact and specific cap 
values I can provide exactly what I used.

The input circuits were to me the biggest pain, but were of critical 
importance.

73,

Jon
KE9NA

-------------------------------------
Jon Ogden
KE9NA

http://www.qsl.net/ke9na    <--- CHECK IT OUT!  It's been updated!!!!!


"A life lived in fear is a life half lived."


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