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@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@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@jeremy.mv.com>
Subject: Re: [Amps] Vacuum variable hipot
To: <donroden@hiwaay.net>, <amps@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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