It has always appeared to me that normal well documented arcs
and sparks in tubes have been used to sell nichrome.
Terry's amplifier very well could have been the victim of just such a
common effect, but since it is not nearly as well publicized outside
engineering circles as parasites it is overlooked.
Actually the arcs stem from several causes, and with enough
accumulated impedance in the HV system they can damage tubes
and other components.
One problem is outgassing (the non-animal kind). The materials
inside tubes release tiny amounts of trapped gas, and this can
cause an arc inside the tube.
A second is seal leakage (again not the animal kind). The bond
between metal and glass or ceramic isn't perfect, and sometimes
slight leakage occurs.
The third is dielectric failures or contamination of the tube by
materials floating around inside.
A fourth problem is barnacles or whiskers. "Barnacles" grow on
copper (I'm not sure about other materials) when it is in a vacuum.
All of these problems, if not severe enough to ruin the tube, are
cleared or temporarily cleared by arcs.
Let me give an example of how bad this problem is. Almost 70
percent of some new tube types arc on initial turn on, or just after
initial turn on.
In the early 1970's I manufactured 50 3CX3000A7 amplifiers, and
virtually 100% of those PA's faulted in the tube after HV was
initially applied. I knew that because the supplies were spec'ed to
have an instant shut down, you could take a grounded screwdriver
and actually touch the anode and hear a loud "tick" before the
supply faulted and dumped power into the dump load.
Around the end of Eimac's run of 3-500Z's, I received a batch of 100
tubes that arced at anything over 8000 volts. They were all bad,
useless in PA's because peak anode voltage can be much more
than two times the dc supply voltage under conditions of mistuning.
Most recently, almost 70 percent of brand new 572B's arc on initial
turn on or shortly after initial use. About 1/3 never heal, even after a
period of anode dissipation to activate the gettering agent in the
tube.
What amazes me is no one, outside of engineering circles, seems
to know these things happen. When someone gets a normal arc,
they blame it on a parasitic.
73, Tom W8JI
w8ji@contesting.com
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