on 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.
I don't see anything wrong with the first statement.
The idea an amplifier with enough feedback to be an uncontrolled
oscillator would sit there stable for months or years and the
suddenly burst into an oscillation that has so much self-sustained
feedback that causes an explosion is based on a ridiculous effort
to make any failure look like it had one root cause.
> 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.
Then you better read Richard Measures claims.
1.) A photon from space can strike a cathode and start a current
avalanche that will arc the standby relay contact over and the tube
will break into a wild parasitic oscillation and cause a high voltage
arc.
2.) A gold-coated control grid can take an almost unlimited amount
of grid current without failure, but a sudden VHF oscillation that
won't even show on the meters will heat the gold enough to ruin the
plating.
3.) (The latest one here) A VHF parasitic will destroy the inside of a
resistor, and not the outside layer of the element, with heat
damage while lower frequencies mainly destroy the outside.
4.) A tube that can only emit so many electrons can, from a
parasitic, suddenly bend a filament helice but it CAN'T do the same
from other causes.
5.) The voltage developed across a few ohms of reactance (the
plate tuning cap at VHF) can get so high when driven by a tube
(see the current limitations mentioned in 4) that the tank can arc
components that won't arc at HF.
6.) A healthy tube that won't arc at HF in normal operation will
suddenly arc at VHF from a spurious signal.
This whole parasitic thing is a major exercise in bad science and
bad physics. I suspect it is designed to make a few bucks selling
snake oil to suckers, or to woo a fan club of people who just don't
quite understand how things work and are looking for one simple
answer to complex questions.
73, Tom W8JI
w8ji@contesting.com
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