[Johnson] Neutralization vs. parasitic suppression, much more informaton is needed.

[Sherrill Watkins]

Dear Richard: Thank you very much for your helpful comments. Will you please
explain exactly what is happening in a screen grid tube that requires it to
be neutralized? Are the electrons striking the plate with such velocity that
they are bouncing off and somehow getting back to the control grid? If the
control grid is negatively biased, how could this happen as the electrons
would be repelled? I do not understand that? I have read that neutralization
takes a small amount of output rf and feeds it back to the input out of phase
but what else is happening to cause this to be necessary?  Also, please
explain what is the cause or source of VHF parasitc oscillation and how does
one distinguish between a neutralization problem or a parasitic oscillation
problem? Can one have max. power output at the plate current dip and still
have parasitic oscillations? How does one know if this would be occuring? I
look forward to your answers. Thank you for your kind assistance along with
Glen and others!! (Information of this nature is not readly available.)-
Sherrill W. k4own.  

-----Original Message-----
From: Richard Peterson [mailto:zapp11 at hotmail.com] 
Sent: Wednesday, September 29, 2004 9:45 PM
To: johnson at mailman.qth.net
Subject: [Johnson] Neutralization


Let's be careful here that we not confuse curing VHF parasitics with 
neutralization -- these are apples and oranges.

Regardless, it was said in some of the older ham literature that screen grid 
tubes were awesome inventions because neutrtalization was not required -- 
the screen isolated the elements of the tubes. That is true to some extent 
-- but I have found that most rigs with tubes such as the 6146, or even a 
4-400, needed neutralization.

Now, how do I know this? There is a simple way to find out: If the maximum 
power output does NOT occur at the plate dip, the final likely needs 
neutralization, or the neutralization requires adjustment. On the other 
hand, if the plate dip and maximum power output occur at the same point, 
then you either do not require neutralization, or the neutralization circuit 
is properly adjusted. In truth, I have found it is possible to operate rigs 
that are not "on the money" in this regard.

I hope this helps.

- Richard, WB5NEN

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