[AMPS] Input tuning

[Jon Ogden]

>KK5YY Jerry the new guy here with a question. I have read of a technique
>for tuning the inputs of Pi input amps that involves removing the tubes
>and
>replacing them with a non inductive resistor then tuning with an antenna
>analyzer.

I am one of the advocates of this method.  A couple of points:

1.) Do not remove the tubes.  You will need them because they have a 
certain capacitance from cathode (or grid) to ground that you will need 
to take into account while tuning.  Just don't power the tube up.

2.) My approach would be to take your total real resistance and multiply 
it by the number of cathode pins.  What I mean by this is if you have two 
tubes, you have four cathode pins.  Multiply the resistance (54 Ohms as 
stated for a 3-500z pair) by four and place the resistors at each cathode 
pin.

3.) I did not use non-inductive resistors.  I just used standard Rat 
Shack resistors for the testing.  Making the leads as short as possible 
will help.  However, remember that the innnards of a tube has some 
inductance as well, so I don't think the inductance of the resistors 
hurts you that much at HF freuencies.

There are others who don't like this method and suggest tuning the 
network for minimum VSWR with variable components, measuring those 
variable components and then replacing with fixed values.  This works, I 
am sure but I think it more difficult.

Have fun.

73,

Jon
KE9NA


--------------------------------------------------------------------------
Jon Ogden

jono@enteract.com
www.qsl.net/ke9na

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


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