It all depends upon what you mean by gas and what is its density.
Altho the air we breathe is a gas it is not called that in tube technology
and to avoid the misinterpertation it is referred to as "going to air" such
as with a seal leak or actual breakage.
A serious air leak will destroy the filament while a minor one will show the
telltale color right at the filament or cathode. If the HV applied to a
minor leak is well below the witholding ability of the tube and the
filaments critical point, that glow is all there is. A 3-500Z with 2500V
applied to a tube that will hi pot to 12KV or more wont break down as long
as the real "gas" isnt also present to any great amount.
If there is an additional gas glow between grid and anode then an arc could
easily occur.
I wouldnt trust any tube that doesnt hi pot to at least 3X the operating
voltage and AM already peaks at 2X plus the modulation index above 1 so 4X
is the mnimum safe point
A gas presence can arc at as low as 1500V or so or not even at 2500V yet in
a 3-500. There will not be any difference in plate current until the actual
plasma arc occurs which will often continue until the HV is completely shut
and bled off. Often applying RF will trigger it which is why those who try
running a tube out of resonance as an "easy" way to increase anode color at
a lower voltage, say 1800V as in many amps with a CW/SSB switch, get a nasty
surprise. An outgassed tube can often be regettered by running HV in the
1000-1200V range and rebiased to draw sufficient plate current to get a nice
bright to brick red anode glow. There is no guarantee a particular tube will
regetter sufficiently to run at 2500V or more but Ive had about a 60-70%
success rate over several decades. Some that wont hold up at say 3500V will
last for many years in a SB-220 for instance.
Gas as referred to here is the result of the manufacturing process which is
recombined into the anode by a gettering process as one of the last
manufacturing steps. Over time and also with prolonged excessive dissipation
the gas will be re-released.
Ceramic metal tubes react similarly but not visibly and the gettering is
slightly different. Regettering often requires just running the filament
from hours to days.
One way to avoid all this gettering issue is to beat on the tube regularly
to keep it gettered. The original 1986 Eimac 3-500Z's in my LK-500ZC were
regularly run in serious contesting and more recently as an AM linear.....if
I have to run up the electric bill I might as well enjoy doing it!
Carl
KM1H
----- Original Message -----
From: "Kimberly Elmore" <cw_de_n5op@sbcglobal.net>
To: "Amps group" <amps@contesting.com>
Sent: Monday, December 01, 2014 1:34 PM
Subject: Re: [Amps] Holding a vacuum
I always thought that a gassy tube is essentially a short when HV is
applied, or perhaps like a thyratron: once current is allowed to flow by
the control grid it can't be stopped until the cathode line is opened.
Gas-filled rectifiers, such as the 3B28, 866A, etc., or the gas-filled VR
tubes are examples. Is there an "in between" state where a tube is gassy
and works fine except for excess plate current?
Kim N5OP
________________________________
From: Bill Turner <dezrat@outlook.com>
To: Amps group <amps@contesting.com>
Sent: Monday, December 1, 2014 11:38 AM
Subject: Re: [Amps] Holding a vacuum
------------ ORIGINAL MESSAGE ------------(may be snipped)
On Thu, 27 Nov 2014 19:15:08 -0500, you wrote:
What metric does one use to determine if a given tube holds a vacuum or
not?>
Cheers,
Karin K3UU
REPLY:
Tubes that lose vacuum draw excess anode current even when idling and
if they are glass, one can see a blue glow in between the elements
when HV is applied.
73, Bill W6WRT
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