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Re: [Amps] grid resonance

To: "Ian White GM3SEK" <gm3sek@ifwtech.co.uk>, <amps@contesting.com>
Subject: Re: [Amps] grid resonance
From: "k7fm" <k7fm@teleport.com>
Reply-to: k7fm <k7fm@teleport.com>
Date: Mon, 7 Aug 2006 06:49:51 -0700
List-post: <mailto:amps@contesting.com>
Ian said:

"The grid in a "grounded grid" amplifier is a case  in point. It is
behaving as an inductive length of wire, terminated inside the tube by
some distributed capacitance to the anode and the cathode... which in
turn are connected somehow back to ground. Certainly you have a circuit
there, with inductance and capacitance in the loop, so it will have a
resonance; and if you can couple a GDO into it, you'll see a dip.

But that resonance belongs to the whole circuit, involving the anode and
cathode and all their associated components. There are too many unknowns
in that loop to understand what the frequency of the observed dip might
be telling us about reverse feed-through from the anode back to the
cathode.

>I must admit that I've never had a lot of use for GDO

Me neither, because it's so difficult to understand what a dip actually
means, except in the very simplest cases.

The GDO is deceptively simple instrument. The trick is to tell when it
stops being simple, and starts being deceptive."

In the past few days, at one extreme, a number of learned gentlemen have 
suggested that they neither use nor understand the dip meter, and that the 
use by others is easily understood.  They suggest complex, expensive 
instruments to evaluate circuits.

At the other extreme, it is suggested that the dip meter is used as a basis 
that tubes, as wired in normal amplifiers, have internal circuits that 
resonate at 80-120 MHz, and that this is the root of the unwanted parasitic 
oscillation.  This conclusion has been reached through the use of a dip 
meter measuring the grid, as connected, of Power Amplifier tubes.  As I 
recall the sequence of revelations, the first article on the parasitic 
solutions appeared in Ham Radio.  However, later articles by the same author 
admitted that the cure did not work, but came up with a real cure - all 
based upon the results of the dip oscillator.

With the publication of the later articles on parasitic solutions, through 
the use of magic wire, life became simplified for the average ham.  All one 
needed to do was incorporate the magic wire into an amplifier, plus a few 
other magic items, and the grids will be tamed - as proved with a dip meter, 
and all generally would be well.

I looked up the word religion in Wikipedia and found the following.  Note, 
because religion encompasses a lot of things, I have removed some words that 
were clearly not applicable.  However, the definition has no added words:

"Religion is a human phenomenon that defies easy definition. It is commonly 
understood as a group of beliefs or attitudes concerning an object (real or 
imagined) or system of thought considered to be highest truth, and the 
practices associated with such system of thought."

It occurred to me that the adoption of the use of the dip meter and magic 
wire to solve the mysterious parasitic phenomena is a religion and it 
simplifies life a great deal.  It means that the only piece of test 
equipment you need is a dip meter (preferably a Kenwood dip meter).  It also 
eliminates any testing at all in the amplifier, and reduces design to a 
minimum, simply by the use the magic wire.

Now, a number of learned men are challenging this simple basis of life, 
taking the position that we should abandon our dip meters and suggest that 
we buy vector network analyzers, spectrum analyzers with sweep generators or 
perhaps only a simple network analyzer - suggesting that life is much more 
complex than another leader has promoted.  One has even taken a position 
that parasitic suppressors are not even required, while at the same time 
actually building and selling an amplifier without one.  Heresy, I say.

Centuries earlier, life was simple, when all of the planets and suns were 
found to revolve around earth.  Then, some heretic with fancy equipment 
claimed that the simpler system was flawed.  He developed a much more 
complex system, which soon brought others over to his side.

I have seen lately that in a number of fields certain people have proclaimed 
themselves to be ethicists - who are able to judge others, and their 
actions.  Seems like a noble field, and some even get paid for such a title. 
It seems like a good field because it allows you to judge others, using 
whatever you feel like using as a basis.

What I am proposing is that I become a parasitic ethicist.  The only problem 
is that I am not sure what I should be studying or what I should adopt as a 
standard.  And, although I have no clue what I am doing, I can still see a 
benefit having an ethicist on the panel, to make ethical and moral decisions 
on such a lofty subject as parasitics.  I can also see the need for an 
ethicist on the use of a dip meter, but I, as yet, have no clue on how I 
would rule on that one.

I am also still trying to understand why Ian and Peter sometimes stay up 
into the middle of the night to participate.  It seems that they are trying 
to stay one step ahead of the others, doing their thinking and writing while 
we are sleeping away.

73,  Colin  K7FM





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