I said I needed to quit this, and here I am.
I wrote:
> >It would be a design challenge, if not totally impossible, to build an
> >amplifier that oscillates only when hit with a sharp pulse and yet
> >behaves normally with a normal signal.
Jon said
> True. Although, I will say this: The output impedance of a tube changes
> with the of anode current. Effectively, output impedance varies with
> output power. It could be possible that a design could be built where the
> amp could become unstable at the power level of the output of the pulse.
> The amp would have to be unstable at that particular value of anode
> impedance that corresponds to the peak of the pulse. It's not impossible,
> but would be very difficult.
The trend is gain goes UP as drive is reduced. That's why were
don't have to hit our oscillators with hammers to start them, and
why amplifiers gain compress at higher power levels.
Try this with you PA. Drive it with 30 watts and tune for maximum.
Calculate the gain.
Now drive it with 50 watts and calculate the gain.
> >Rich's theories rest on the very shaky foundation that the PA is an
> >uncontrolled oscillator with feedback levels high enough to send it into
> >meltdown only on "rare occasions", when hit by cosmic bullets or
> >disturbed by a relay closure.
>
> I get that same impression. It seems that the belief is that no
> amplifier is always stable and every amp is always waiting to go into
> oscillation. It's what I call the nasty lurking parasite creature theory.
> Parasites are invisible creatures that live in PAs. They come out when
> they get angry and they do bad things.
People love simple answers. When the correct answer is complex,
many people take the shortest answer even if it is technically
incorrect. Everyone who knows the absolute answer is an expert,
and if the absolute answer is one word everyone becomes and
expert.
Take for example tube arcs. Tube arcs come from outgassing, gas
ingress, or internal insulation problems. When the arc occurs, the
plasma from the arc getters the tube. (Tube engineering books
clearly state arcing is one way a tune getters itself.)
Some tubes are intentionally built to "recycle" gas. For example,
the zirconium used in high power transmitting tubes. At 1400
degrees it absorbs large amounts of various gasses. It gives off
hydrogen however when heated beyond 350 degrees C.
The tube is designed to operate with zirconium at two
temperatures. That's why glass tubes stored for long periods often
arc at power-up, and why operating a zirconium gettered tube at
low anode power (3-500Z) will eventually ruin it.
None of this stuff is magic, although the real explanation for
occasional arcs is much more complex than "parasitic". Parasitic
is an easier word to remember and parrot than "outgassing, gas
ingress, or dielectric failure". It just sounds knowledgeable!
> In Rich's defense, I must say though that he does have some good stuff in
> general amp design and his idea for stabilizing the cathode of an
> amplifier works quite well. I am not sure it's his original idea but it
> is what he suggests.
Interesting. Since the cathode is not the major player in VHF
oscillations in traditional GG amps, I wonder how de-Q'ing the
cathode would stabilize the PA?? Or are you talking about HF
oscillations, that occur in tubes with large amounts of feedback
capacitance at HF?
73, Tom W8JI
w8ji@contesting.com
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