Hi Terry,
I wrote:
> > If your 3-500Z was operating at six volts and 17 amperes (over the
> > ratings) the filament power would be 102 watts. That would be a
> > maximum available emission of 10.2 amperes for an ideal filament
> > structure with maximum emission.
You replied:
> I understand where you are coming from here. But the fact remains it
> happened not only once but twice ... just as I stated.
This is where we get into a "guessing session". I tried to give an
example by telling the true story of my sister-in-law, who believes
in some stuff called numerology...where numbers control your
destiny.
She had two husbands in a row die sudden deaths after she moved
into houses with numbers that "added up" to bad numbers. There
is no convincing here it was coincidence, because she then says
"hardly anyone has one husband die, let alone two".
All folklore myths are based on similar evidence. She won't
consider how many other people have "bad numbers" but who's
husbands don't die.
I can't say why your tubes fail, except to point out the following:
1.) Photons don't have enough mass and energy to cause a large
spike in current. They can't knock enough electrons loose to make
a measurable change in conduction in a vacuum tube. Even tubes
designed specifically to detect them have low-pressure gas and
operate at high voltages, and the current in them is insignificant in
a system like we are talking about.
2.) Even if they did knock an electron loose, it would be lost in the
large cloud of electrons boiled off the filament that surround the
filament.
3.) Even if they did stand out in a crowd of other electrons, they
would have to make it past the large negative electric field of the
control grid that is holding all the other electrons back from the
anode.
4.) Even if they did reach the anode, they would have to do so in
massive numbers to force the cathode to negative rail voltage to
charge all the bypass capacitors to a few thousand volts and arc
the relay. You can't do that with a 100 watt exciter, do you really
expect photon's to have that much energy? If so, don't walk outside
of your salt mine!
5.) Even if they reach such massive numbers to arc the relay, now
you need to have an oscillator that never occurred in normal
operation that has so much feedback it drives the tube to hundreds
of amperes of grid current in a few nanoseconds or so.
6.) Even if the oscillator had that much feedback but somehow
magically remained stable in regular operation, it would have to
force the tube to have more emission than the tube is physically
capable of!
Either the laws of physics are all totally wrong, or you had
something else go on. One possibility is a bad weld letting a grid
wire hit the anode, and dumping the anode supply into the grids.
Are the grids solidly grounded, or are they floating?
What about a grid wire hitting the filament and anode?
What about a high-vacuum arc through gas or contamination in the
tube(s) that dumps the HV to the grid without any current limiting in
a sustained dc arc?
What if the tubes were ready to break a weld, and when you moved
the PA and took the cover off they just let go?
You see, many other things are much more likely than something
that is impossible for many many reasons that we can be sure are
all true.
These stories about parasitics are indeed nice ones, they all have
a tiny grain of truth in them that is woven into an all out
fantasy...and that's what makes them such good stories.
We just have to keep in mind how things really work.
73, Tom W8JI
w8ji@contesting.com
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