[AMPS] Modelling and its failures
Wed, 13 Sep 2000 09:40:40 -0400
From: "Fred Fliss" <firstname.lastname@example.org>
Subject: [AMPS] Modelling and its failures
Date sent: Wed, 13 Sep 2000 06:09:49 GMT
> 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
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
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
> 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
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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