Carl Clawson wrote:
>
>> (Tom)At idle, a class AB amplifier is in class A so it indeed is linear
>
>Agreed, Tom. We're more in agreement than otherwise. My point is that, given
>sufficient excitation, a nonlinear system in general can exhibit a
>self-sustaining oscillation. The "sufficient excitation" bit is what brings
>the nonlinear behavior into play.
>
I liked your analogy of the motorbike engine - it certainly kicked me
from thinking "can't ever happen" into "could happen sometimes."
However, the motorbike engine is pretty predictable [insert joke here].
When stopped, it is conditionally stable (to use the electronics term)
and we know what the minimum energy input for instability is - in other
words, how much kick it needs to start it. On the other hand, once it's
running, it is also a completely understandable stable oscillator that
is limited by fuel input and the applied load.
The difference with tubes is that when they're biased to cutoff, we
can't think of a credible mechanism that might kick them into
sustainable oscillation. Rich's proposed mechanism with cosmic rays
starting an electron arc is stretching credibility too far - there is no
electron-multiplication mechanism in a vacuum tube that could be
equivalent to either a Geiger counter (which is not a vacuum tube - it
works by ionization of low-pressure gas) or a photomultiplier
(completely different structure - multiple cascaded cathodes).
For random and very rare events, I'm still inclined toward the gas arc,
triggered by very slow migration of trapped gas to the surface of the
metal structure. This is a known and understandable mechanism - the only
thing we don't know is when a particular gas concentration will diffuse
to the surface and be released.
Terry's case is different because it seems to be reproducible... well,
it's happened twice under the same circumstances. (Come on Terry, three
times is a charm - aren't you willing to sacrifice a few more 3-500Zs in
the cause of science?) But seriously... I think Jon is right in saying
that raising the lid is going to sweep the VHF/UHF resonances in a way
that the designer never even thought of, and never protected against.
Thinking further, for oscillation we need (a) an output-side resonance
and (b) a feedback path with a loop gain of at least 1 in the correct
phase. The resonance is always with us - in fact the output circuit
always has a choice of at least two resonances, the designed one at HF
and the VHF parasitic resonance. There will be a third "cavity"
resonance at UHF due to the box and cover, and the presence of the
metalwork can also serve to tune the other two. No shortage of output-
side resonances, then.
What about feedback paths in a GG amp? These too are frequency-dependant
and will depend on such things as the natural resonant frequency of the
grid, the inductance of internal and external leads, and any external
capacitors. Somewhere at VHF/UHF there is going to be a frequency where
the grid-to-ground circuit becomes parallel resonant and the feedback
path is wide open. If you "sweep" the output-side resonance by gradually
opening the lid, it might be possible to hit that feedback path
resonance. If the loop gain there is greater than 1 (in the correct
phase), it will oscillate.
The tube being in "standby" bias is a problem, of course, but I wonder
if the bias actually reduces the gain of the tube below 1? Switching the
input/output relays leaves the tube sitting there with no input or
output load. If the gain of the tube in this configuration was even only
slightly above 1, it would still be capable of starting an oscillation
that would build until some limiting mechanism kicks in - like part of
the circuit being physically destroyed.
The fact that the grid structure was destroyed is very suggestive of a
grid resonance being involved, in Terry's particular case. That isn't to
say the same is true of all similar events, but anything that physically
damages the grid must by definition have deposited a large amount of
energy there.
>>
>> Class A is the point of highest gain.
>
>
>For small signals, yes. For large transients of arbitrary shape, that would
>be hard to prove. The stability of nonlinear systems is very difficult to
>evaluate. Generations of engineers, physicists, and applied mathematicians
>have spent their careers on the subject. And it isn't just an academic
>exercise either. I recall chaos being a real problem in some experimental
>parametric amplifiers that some folks were working on back in the early
>`80s.
>
What you say is true, Carl, but unfortunately it fuzzes the whole topic
out into the realms of hand-waving. There is no real evidence that the
normal behavior of vacuum tubes is in any way chaotic (ie is inherently
unpredictable) - very much the contrary.
Pathological behavior of vacuum tubes may be a different matter, but if
you suggest non-linear or chaotic behavior, you also have some
responsibility to explain *how* that might be so. Otherwise you turn the
whole subject area into a happy hunting ground for bad science - which I
know is the exact opposite of what you intended to do!
As far as I can see, the only pathological mechanism that steps right
outside the normal explanations of vacuum tube behavior is the gas arc -
but I'd be very interested to see suggestions of others.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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