[Amps] Vacuum variable hipotting

[TexasRF at aol.com]

John, perhaps you can shed some light on how bad can a vacuum leak be  
before the tube is unusable.
 
I have seen some used tubes that begin to show current with as little as  
1600 volts between the anode and screen grid. I have assumed there is little 
or  no vacuum in that case and moved on. Perhaps this is a wrong assumption?
 
73,
Gerald K5GW
 
 
 
 
In a message dated 9/18/2009 4:40:29 P.M. Central Daylight Time,  
jtml at losalamos.com writes:

Yes, you  aren't really cooking the vacuum, you are 'spark knocking' the 
device. By  applying controlled HV, the pesky whiskers that tend to cause 
sparkovers  inside the device will be emitting electrons, cold emission from the 
enhanced  electric field around the sharp point. The current flow from the 
power supply  with dump some power in these points and they burn away. Or 
they get blown  away from sparkover if the energy is high enough. This is 
essentially how a  high potter can improve a vacuum device, not by sitting there 
at zero leakage,  but when the leakage is showing, its working. Over time, 
the leakage should  diminish and the voltage is then increased again, still 
within the ratings of  the part. If the voltage is high enough, typically 
over 10 kV, weak X-rays may  start being measurable. On higher voltage big 
tubes, they become quite  objectionable, and require safety practices such as 
shielding. I have observed  tetrodes drawing several mA at 50 kV and 
generating copious  X-ray
s,
without even having filament connected. If the leakage doesn't  drop over 
time, then one might suspect the vacuum having a small leak, or a  major 
sharp point inside that cannot be cleared off. Or just dirty ceramic or  glass 
insulation. 

73
John 
K5PRO

Message: 6
Date: Fri,  18 Sep 2009 11:17:31 -0500
From: donroden at hiwaay.net
Subject: Re: [Amps]  Vacuum variable hipot

Can you "really" cook a vacuum ??
I have never  heard of a getter function being available.
Don  WA4NPL

------------------------------

Message: 7
Date: Fri,  18 Sep 2009 13:09:49 -0400
From: "Carl"  <km1h at jeremy.mv.com>
Subject: Re: [Amps] Vacuum variable hipot
To:  <donroden at hiwaay.net>, <amps at contesting.com>
Its something to  do with zapping off whiskers iff'n I recall correctly.
Similar to what  Eimac does with "real  tubes"

Carl
KM1H

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