On Mon, 06 Jun 2005 15:05:43 GMT, "Henry Kolesnik"
wrote:

Please tell me more about melting the finals and a bit more explanation of
what was happening?

Hi Hank,

Direct observation offered a glowing plasma between the filament and
the plate. It was football shaped rather than beam-like or an
amorphous cloud. One point of the football touched the cherry red
plate. Following a quick power-down, that point on the plate did not
exist anymore as there was a hole. Couldn't really tell, but no doubt
the grid suffered just as much in its own way.

I suppose plates have become more robust over the years since that
amazing demonstration. I helped fix one friend's Amp when it failed
along with his antenna (or t'other way round as the chain of causality
would suggest). His Amp simply quit working suddenly during bad
weather. Fuses checked - OK. No interlocks were open - OK. The tube
looked good at a glance - OK. HV Supply looked good - OK. The
filaments failed to light up - odd, but consistent. Time to crack
open the case. Pulled the tube and measured its filament continuity -
OK. Measured filament supply - OK. Things were really getting
strange. Time to bust the chassis open and really look. There on the
baseplate were several small pools of solder in a circular pattern -
how odd. Close scrutiny of connections revealed bright and soldered
wires to everything - OK. Time to look at the tube again. Every pin
was solder free - that was on the chassis base plate. [dirge played
here] The filaments' wires were making enough contact to measure
continuity, but no where enough to support real power.

In other tubes I've seen the heat become so extreme that the glass
envelope slumped into the vacuum and enclosed the plate structure like
taffy. This didn't even crack the glass (or it had simply re-fused).
Tube still worked afterwards though (so I would suppose the glass
never cracked). One occasion was actually due to a bias problem
created when the cathode load shorted. Lack of bias protection sent
the circuit into massive conduction. Of course the short came about
because of an initial excessive conduction (surprising in its own
right because the common failure mode is to open).

I've also seen stressed thyratrons so mismatched that they filled the
workspace with their purple glow like a floodlight. Thankfully fuses
work as I did not want to be near that final testimony.

Now, that was the short list of Bottle failures. I have another list
of melted state failures too, but their evidence is usually better
hidden and less dramatic - heat sinking generally spreads the risk,
so to speak. And speaking of heat sinks, I've drawn a number of
blisters from those that normally only warmed my hand. Note there
should be emphasis on normally warm and the obvious contradiction with
blisters.

Experience with failure has strongly correlated with heat and
mismatch. Heat was born by resistance. Resistance is part of life
and amplifiers. Heat comes in two forms. Slow-like, which is
generally current based; and sudden, which is generally voltage based.
I've felt along heatsinks immediately following failure that were as
cool/tepid as usual, or ominously cooler! The sudden heat of arc-over
in silicon can destroy just as effectively as the long slow broil of a
plate turning to slag.

Every mismatch in the Amateur experience is a probablistic spin of the
wheel of misfortune. Sometimes the wheel stops on the slow bake that
aborbs into heatsinks and you notice unusual smell, or your fan
running on too long - Quick! do something, and you survive. Other
times there's the snap of finality. Both of these examples are for
those with keen senses, and often failure comes as a whimper.

Most suffers usually discover what matching is for, even if they don't
know how it works.

73's
Richard Clark, KB7QHC