> resistance alone may work. But the cause of an unexplained discharge
> within the envelope of a vacuum tube is NOT always because of an
> undesirable oscillation.
Actually, you can go further than that. Arcs inside tubes are almost
always NOT caused by parasitics.
In order to have an arc, we all know the peak voltage between two
points has to exceed the breakdown voltage. The highest peak
voltages normally occur at the frequency where the amplifier is
designed to operate.
While an oscillation can occur on frequencies other than the
operating frequency, the most likely frequency to trigger an arc is a
frequency at or near where the circuit is optimized for maximum Q
and gain or below that frequency where anode load impedance is
The tube should be the last component to break down. A good tube
typically has many times the rated dc voltage for breakdown
voltage. It is virtually impossible for any self-oscillation to cause a
tube with normal voltage breakdown to flash over, just as it would
be to trigger the arc with actual drive power.
What typically does cause a "glitch" is outgassing in the tube as
elements heat, seal leakage allowing air in, or metallic "whiskers"
or debris inside the tube. In most cases, the arc itself will remove
the problem. If it is a slight outgassing, energy in the arc will break
down the gas and getter the tube. If it is a metallic whisker or
debris in the tube, the arc will normally vaporize the stray material.
Tube arcs most often occur either at application of high voltage, or
during a period of maximum anode voltage (when drive is applied
with light loading).
Certainly a parasitic can trigger an arc in an unhealthy tube, but far
more often it is application of HV from a cold start or application of
drive at the desired operating frequency that triggers the arc. But in
all cases, a tube with normal voltage breakdown will NOT arc...even
if it would by some rare chance be oscillating.
Of course glitches can occur outside the tube also from dust,
moisture or other circuit problems.
73, Tom W8JI
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