>
>> 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.
>
// True, Mr. Mr. Rauch. Big-bang arcs undoubtedly take place in the
atmosphere.
>In order to have an arc, we all know
// Mr. Rauch has a frog in his pocket?
>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.
>
// What if an amplifier was generating energy above the HF passband of
the tank, where the energy produced could not be dissipated in the load?
>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
>high.
>
>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.
>
// Good point.
>What typically does cause a "glitch" is outgassing in the tube as
>elements heat, seal leakage allowing air in, ...
// However, such gas seems to vanish just before the suspect tube is
affixed to a high-pot tester.
> ...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
>W8JI@contesting.com
>
>--
>FAQ on WWW: http://www.contesting.com/FAQ/amps
>Submissions: amps@contesting.com
>Administrative requests: amps-REQUEST@contesting.com
>Problems: owner-amps@contesting.com
>
>
- R. L. Measures, 805.386.3734, AG6K, www.vcnet.com/measures.
end
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