[ARC5] [Boatanchors] More and More "Gassy" Tubes

Mon Apr 21 13:58:36 EDT 2014

> Let me see if I understand this correctly.  I think you are saying that
> for
> N2 in particular, that molecule needs to be ionized in order to "stick" to
> the getter surface.

They don't 'stick'. That's sputtering. That only happens in BIG tubes with
external pumps.

The ionized N atoms chemically combine with the Ca or K of the getter.

> But since the getter surface does not have an applied
> potential (it floats on the glass envelope), there is no significant E
> field potential to attract those ions.

The doesn't need to be a high e-field to cause the alkali netal and N ions
to chemically combine. The field is needed to make the ions.

> Meanwhile there are tube electrodes
> nearby with strong attractive fields if the tube is biased for Ip.  How do
> the N2 ions find their way to the getter flashing?

When an e- strikes a neutral N atom, the N ion is attracted to the
cathode. SWome get to the nearby getter and combine there. If you try to
degas a tube with too high a current flow, the cathode gets poisined and
the Gm goes way down.

> I am not disputing what
> you are saying; you have done the experiments involving N2.  I just don't
> fully understand the role of the ionization in the N2 example.

The only way to pump neutral atoms is with sputtering...  basically you
plaster the gas atoms to a surface with metal atoms. Pumping ions is much
easier.

If you really want to find out what is going on, look at the glow with an
optical spetrometer. If the gas in the volume of the tube is glowing,
you'll see the elemental spectral lines. I*n fat, you can tell what gas is
 in the tube.

-John

=================

>
> Dennis AE6C
>
>
> On Mon, Apr 21, 2014 at 10:16 AM, J. Forster <jfor at quikus.com> wrote:
>
>> You can store metallic Na or K under N2.  NOTHING happens. For decades.
>>
>> You need the ionization to getter the N2. Do the experiment. I have.
>>
>> -John
>>
>> ======================
>>
>>
>>
>> > Actually, ionization is not a factor in the operating mechanism of a
>> > getter.  A getter is composed of a highly volatile and reactive metal.
>> > The
>> > unwanted gas molecules are agitated in their random motion by
>> application
>> > of heat.  Just the heater alone is often sufficient but application of
>> > some
>> > Ip will increase the heat and also expose more of the tube elements to
>> > elevated temperature.  This increases agitation of the molecules which
>> in
>> > turn increases the rate at which they eventually get captured by the
>> > getter.  If a getter depended upon ionization to do its job then there
>> > wouldn't be a white coating on the getter when the tube envelope
>> cracks
>> > and
>> > lets air inside.
>> >
>> > Dennis AE6C
>> >
>> >
>> > On Mon, Apr 21, 2014 at 9:51 AM, J. Forster <jfor at quikus.com> wrote:
>> >
>> >> You need some Ip, IMO. If there is no electron flow, the gettering
>> >> process
>> >> does not work properly.
>> >>
>> >> It takes an electron flux to ionize the neutral gas atoms. W/o the
>> >> electrons, no ions.
>> >>
>> >> Neutral atoms are not attracted to anything.
>> >>
>> >> -John
>> >>
>> >> ====================
>> >>
>> >>
>> >>
>> >> > Residual gas from sitting on a shelf is very common, even with
>> tubes
>> >> of
>> >> > relatively recent manufacture.  For example, when I replaced a set
>> of
>> >> > 6146B
>> >> > finals that were purchased fresh in 1980 and installed in 2005, I
>> >> > neglected
>> >> > to give them a little "burn-in" before firing the rig back up for
>> the
>> >> > neutralization.  My impatience was punished with a couple of
>> >> > inter-electrode arc's at first.  Then I just stopped and left the
>> tube
>> >> > with
>> >> > heater and HV applied (no Ip because it was easy to run it this
>> way)
>> >> for
>> >> a
>> >> > few hours.  Although the tube wasn't very hot, it was sufficient to
>> >> > activate the getter and clean up the residual gas.  After that
>> little
>> >> > exercise the tubes performed flawlessly.
>> >> >
>> >> > Dennis AE6C
>> >> >
>> >> >
>> >> > On Mon, Apr 21, 2014 at 8:53 AM, Bill Cromwell
>> <wrcromwell at gmail.com>
>> >> > wrote:
>> >> >
>> >> >> On 04/21/2014 09:58 AM, J. Forster wrote:
>> >> >>
>> >> >>> Tubes are not perfectly sealed, and some gasses (He especially)
>> >> >>> diffuses
>> >> >>> through glass. That's the bad news.
>> >> >>>
>> >> >>> The good news is many can be saved. That's what the getter is
>> for.
>> >> BUT,
>> >> >>> the tube MUST be operating for the getter to work. The trick is
>> to
>> >> >>> power
>> >> >>> the tube without destroying it and let the getter to do it's job.
>> >> >>>
>> >> >>> Apply normal filament.
>> >> >>>
>> >> >>> Apply negative grid bias to ensure cutoff through a roughly 100k
>> >> >>> resistor
>> >> >>>
>> >> >>> Apply about 150 to 200 VDC to the plate through a current
>> limiting
>> >> >>> resistor to limit current to roughly 1/2 max Ip.
>> >> >>>
>> >> >>> Slowly reduce grid bias, until Ip is about 20% of design Ip.
>> >> >>>
>> >> >>> WAIT. At least a day.
>> >> >>>
>> >> >>> The above an all be done on a tube tester.
>> >> >>>
>> >> >>> Retest for gas.
>> >> >>>
>> >> >>> -John
>> >> >>>
>> >> >>> ===================
>> >> >>>
>> >> >>>  I have done this and had about 95 per cent success. Obviously if
>> >> the
>> >> >> glass is visibly cracked or the getter looks like chalk or cobwebs
>> it
>> >> >> isn't
>> >> >> going to work.
>> >> >>
>> >> >> 73,
>> >> >>
>> >> >> Bill  KU8H
>> >> >>
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