[AMPS] Modelling and its failures

Tom Rauch W8JI@contesting.com
Wed, 13 Sep 2000 09:40:40 -0400

From:           	"Fred Fliss" <fredfffff@hotmail.com>
To:             	amps@contesting.com
Subject:        	[AMPS] Modelling and its failures
Date sent:      	Wed, 13 Sep 2000 06:09:49 GMT

Hi Fred,

> It has been written that an amplifier either oscillates, or it doesn't.
> That is a half truth.  It is possible that an amplifier is on the verge -
> that is, with a slight change to some parameter, such as valve gain, a
> resistor value, or stray capacitance, it will oscillate.  This is
> ABSOLUTELY a measurable, and predictable condition. 

Which is my point. Please don't take what I say out of the context 
it was in. There is enough at that going on Fred.

My points are:

If you have an amplifier that arcs capacitors, blows bandswitches, 
melts grids.....you can be sure that amplifier can be made to 
oscillate repeatedly. It won't play along for years and years, and 
suddenly because of an exploding star in another galaxy, burst into 
uncontrolled oscillation.

Also, if feedback is so low that the oscillation only sustains with 
excitation, it is unlikely to have a level that does any more than 
cause TVI. It is also possible to make that oscillation sustain 
without drive, through manipulation of bias, HV, and load or source 

>From that stems my statement you either have an oscillator, or 
you don't. Either conditions are correct for an oscillation, or they 
are not.

I received a little "heat" for my comments about what I would do to 
fix a TL-922, when I said replace the bad component(s).

Truthfully, that's all I would do. I'd simply find the bad components 
and replace them. If something is "on the edge", I might add more 

My point here is when someone "shotguns" a PA by replacing a 
bunch of components PLUS waving a "nichrome wand" over the 
amplifier, they have no idea what they did to fix the PA. 
"Shotgunning" might be a common troubleshooting technique, but 
it sure doesn't give any insight into what really caused the problem.

It clearly is meaningless when someone changes a half dozen 
things and then says "the nichrome fixed the problem, it must have 
been a parasitic". They have no idea what fixed the problem, or 
what caused the problem!
 Rich is wrong in this
> matter, parasitics are NOT unpredictable.  Just because they are
> intermittent does not preclude deterministic analysis.

That's right, there are test procedures and measurements that can 
predict problems. 

> If an amplifier analysis shows that there is a forward gain of 13 dB at
> some frequency, and a reverse attenuation of 12dB at the same frequency,
> then you have a potential oscillator on your hands.  I once did an
> assessment of an amateur amplifier which was claimed to oscillate.  Having
> made parametric measurements of the valves involved, two-port measurements
> of the input and output networks, I then adjusted the stray elements in my
> SPICE model, and the model actually showed the amplifier to be 1/4dB shy
> of the gain required to sustain oscillation.  No wonder that if the tune
> capacitor was tweaked the wrong way, it would break into oscillation (at
> the predicted frequency).

Yep, you either have an oscillator or you don't. It won't sit there 
through years of service fully stable with no ill effects and the 
suddenly explode. If it is that close to oscillating, you can easily 
prove it will oscillate.

We had this same hysteria in the 70's. At that time, nearly every 
tube failure was blamed on inrush because one guy wrote an article 
about inrush.

The rage was how stupid Heath and other companies were for not 
adding inrush protection. Well, if anyone had bothered to test a few 
3-500Z's for inrush they would have found even a 50 amp 
transformer with the leads clamped right to the filament pins of a 
single 3-500Z wouldn't hurt the tube after thousands of startup 
cycles. If they would have measured an SB-220, they would have 
found inrush was well under two times the operating current of the 

The problem is, once something gets in text people take it as a 
fact. It's easier to depend on someone to tell them what the 
problem is...especially when one single answer covers all 
failures.....than it is to learn what is really going on.

I sort of consider it the "school of instant experts". One person, 
with little or no test equipment and no manufacturing experience 
who has no access to field failures, gives a technical answer 
everyone can parrot. The result is a cult-like following where people 
quit learning and thinking.

Thanks for your perspective Fred. Mine is that this parasitic stuff 
has become almost like a religious cult.

73, Tom W8JI

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