> >
> > The amplifier was in STANDBY ..... NOT KEYED .... but with all
> > voltages ON ! I had a reason to remove the top cover (high
> > voltage interlock removed), I slowly raised the cover from the
> > front of the amplifier leaving the rear of the cover sitting on
> > the amplifier ... when I got the front of the cover raised about
> > 3 inches ... all of a sudden the amp made this huge big grunt
> > and the circuit breaker tripped off. After I removed the top
> > cover and turned the amp back on I had idling plate current in
> > the standby mode. Long story short ... I had one of the two
> > 3-500Z's with a grid to filament short and that tube had metal
> > particles rolling around inside of it.
>
> Your anecdote missed two important facts:
>
> 1) The amplifier was stable with the top cover on ... you made a
> change, probably in the plate circuit stray capacitance when
> you moved the top cover. That change "tuned" the parasitic
> circuit.
First I would have to hear something that indicates it actually had
an oscillation Joe, other then a guess.
> 2) The amplifier has "all voltages on" ... as such, there was
> undoubtedly some residual anode current (all voltages could
> not have been "on" if the cathode were completely cut off).
> This allowed gain (and oscillation) if the feedback conditions
> were "tuned" properly.
Bingo. A PA that oscillates when the tube is in full cutoff? Not
likely, is it?
> The rest of the argument is simply over the "trigger" to start
> the oscillation. In fact, the triggering event does not matter -
> it can be photon bombardment (Measures),
Can it? Ever look at the amount of photons required to generate 1
mA of current when the strike the cathode of a tube?
Talk about not understanding vacuum tubes and coming up with a
wild theory! The cathode is surrounded with free electrons when it
is hot. They are constantly boiling off.
A photon hitting the cathode would be as dangerous to the tube as
an ant running head-on into a tractor would be to the tractor.
> (your cover removal), a gain shock (HV "spike" due to a mains
> glitch), momentary overdrive (the ALC spike from many modern day
> solid-state exciters) or simply gaussian noise ... some more
> plausible than others.
This reminds me of the disagreement I had with my wife today. Her
sister lost two husbands immediately after she moved into a house
who's street number totalled so "bad number" in numerology.
There is no way to convince them now that addresses have nothing
to do with dying. Even though it makes no sense and has no
bearing on the results, sometimes people "just believe" in
something.
Nothing wrong with that, as long as we don't call it science.
73, Tom W8JI
w8ji@contesting.com
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