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[AMPS] Parasitic Oscillation Repeatability

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Subject: [AMPS] Parasitic Oscillation Repeatability
From: (Tom Rauch)
Date: Mon, 11 Sep 2000 08:58:24 -0400
> Therefore, a parasitic oscillation must be repeatable - even if for a
> brief collection of oscillations.    This definition would seem to
> preclude random, non repeatable events - since they could not be
> oscillations.  And, if they are not oscillations, they could not be
> parasitic oscillations.
> Colin  K7FM

There are two forms of damage Colin.

One is caused by voltage breakdown, the other is heating.

If the parasitic oscillation was a one time event that caused an arc, 
it could ionize an air path and trigger a sustained arc. That would 
only happen if there was something to ionize.

The real flaw in all this parasitic nonsense is much or most of the 
damage is said to be heat related damage. That requires time, not 
just a sudden short burst of energy.

The amazing thing is the "power" to damage grids is supposed to 
appear with any grid current indicated on the meter! How we have 
thermal damage without grid current is not easily explained unless 
some very very poor science is thrown in.

The problem with the arc is in the impedances and voltage 
breakdowns. Having measured the impedances in some amplifiers 
Rich claims arc bandswitches from VHF parasitics, I can tell you it 
is impossible to develop the voltages necessary within the current 
sinking limits of the output device.

Any amplifier that developes such voltages would be a TVI 
nightmare, because the tank would not "short" or bypass the VHF 
harmonics to ground.

Now I admit, for Pete's sake, that it isn't "impossible" to design a 
PA that would produce high tank voltages at VHF. But the dead 
giveaway that was happening would be a look at the spectral purity 
of the PA in the normal testing procedures. If I saw an excessively 
high harmonic somewhere in VHF, I likely could find just such a 

As Peter says, it isn't impossible for his wife to win the lotto. But 
what we are asked to believe, by Rich, is virtually every failure in 
every PA is a parasitic....and some amps have that problem on a 
regular basis.

It is very easy to prove that "theory" wrong.
73, Tom W8JI

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