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[AMPS] amazing doubletalk...

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Subject: [AMPS] amazing doubletalk...
From: (2)
Date: Thu, 13 Sep 2001 12:42:34 -0700
>> 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 

>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 
>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
>FAQ on WWW:     
>Administrative requests:

-  R. L. Measures, 805.386.3734, AG6K,  

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