[AMPS] FAILURE OF PARASITIC RESISTORS IN AL-811X AMPLIFIER

[Carl Clawson]

After some discussion about how oscillators work, someone said:

> If the amplifier is going to oscillate, it will oscillate.  It won't sit
> there and just do nothing for weeks, months, years, etc. and then suddenly
> WHAMMO!

This is a misrepresentation, and I think you know it. The amp is in use, and
drive, impedances, etc are changing from time to time. It's not just sitting
there. The spurious oscillation hypothesis is that particular combinations
of drive, impedance, etc. are deadly.

You then go on to say:

> Too true.  Too bad some people don't understand the laws of physics.

There is no law of physics that forbids the behavior described.

I'm a physicist; I would know.

It is easy to imagine systems that could behave in this way. Strongly
nonlinear systems can exhibit all kinds of strange and wonderful behavior.
Witness the burgeoning field of chaos and nonlinear systems that has
developed over the last 25 or 30 years. Consider:

An oscillator is typically designed by applying positive feedback to an
amplifier that we might call an almost-linear system. It has a constant gain
for low signals, and the gain eventually decreases with increasing
amplitude - if for no other reason than hitting the supply rails. If the
loop gain is high enough this circuit will indeed be stimulated into
oscillation by thermal voltages because the positive feedback acts even for
small signals.

Now imagine a system where for small signals the loop gain instead increases
with signal level. You could arrange this be putting a threshold device or
other strongly nonlinear element in the loop. The system might then be
stable for small excitations yet break into oscillation for large enough
excitations. Now, doesn't this pretty well describe the posited behavior of
the spurious oscillations?

I don't claim that I have shown the spurious oscillation to be the cause of
the big bangs, or that it is a likely explanation in a practical tube
amplifier. I've merely argued that physics does not prohibit it, and that
one wouldn't need to be too awfully clever to design an oscillator that
would need a good swift kick to start up.

Like a motorcycle engine --  which also doesn't violate the laws of physics.

Carl WS7L




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