gasified valves

[N Cook]

What is the white deposit chemically, is it always white, and why does it
always form on the dome end whichever-way-up the valve is mounted.

 

[James Sweet]

What is the white deposit chemically, is it always white, and why does it
always form on the dome end whichever-way-up the valve is mounted.

--

By valve, are you referring to vacuum tubes? The deposit sounds Rilke
the getter, if it's white instead of silver that means air has got in
the tube.

It could also be evaporated material from the cathode deposited on the
inside of the glass, that will form wherever there's a clear path from
the cathode to the glass.

 

[[email protected]]

What is the white deposit chemically, is it always white, and why does it
always form on the dome end whichever-way-up the valve is mounted.

Most tubes commonly in use have a Barium getter. This material is
vaporized within the tube during the manufacturing process to absorb
any loose gas molecules within the tube. When the tube cracks or loses
its seal, the Barium oxidizes quickly, thereby turning white (Barium
Oxide).

There are other getter materials used, but Barium is far-and-away the
most common.

Peter Wieck
Wyncote, PA

 

[Meat Plow]

What is the white deposit chemically, is it always white, and why does it
always form on the dome end whichever-way-up the valve is mounted.

Oxidized mercury?

--
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[Jamie]

What is the white deposit chemically, is it always white, and why does it
always form on the dome end whichever-way-up the valve is mounted.

Hmm, don't know?, could be the heater falling apart?
could be the results of the getter used to remove the
remaining Oxygen.

 

[Sam Goldwasser]

Hmm, don't know?, could be the heater falling apart?
could be the results of the getter used to remove the
remaining Oxygen.

By far the msot common is the getter. Why is it usually on top? Because
that's where the original metallic barium or other highly reactive material
was placed during manufacture. Sometimes it's on the side or near the
base. But since it's a metal and conductive, has to be placed away from
an area where it could short between pins.

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[The Old Guy]

As I remember, the getter used to be a pellet of magnesium. After the
tube or valve was evacuated the magnesium pellet was "fired" by the
use of RF. As the magnesium burned it used up any remaining oxygen
within the tube/valve and "silvered" the inside of the glass (black
and shiny). If the tube or valve cracks or air leaks into it the
gettering on the inside of the glass finishes oxidising and turns
white. This is when we refer to a tube as "gassed".
Roy the old guy.

 

[N Cook]

By far the msot common is the getter. Why is it usually on top? Because
that's where the original metallic barium or other highly reactive material
was placed during manufacture. Sometimes it's on the side or near the
base. But since it's a metal and conductive, has to be placed away from
an area where it could short between pins.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

I'd thought that was silvering to block stray electrons or something firing
out of the tops of the tubes. I'd thought this whitening process was some
sort of safety/failure indicator process as a visual indication of a
mechanically failed valve. Still learning even if an ancient technology

 

[[email protected]]

What is the white deposit chemically,

metal oxides


NT

 

[[email protected]]

OK... let's start at the beginning.

"Getters" are usually alkaline-earth metals flashed onto the inside
surface of the tube during the manufacturing process by a number of
mechanisms that are not terribly relevant, but include inductive
heating, a high current and voltage applied to a pellet of the
candidate metal that vaporizes it or any of several other mechanisms.

These getters are to "get" stray molecules of gas that may remain in
the tube from the manufacturing process. So, they are going to be very
'active' metals.

Far-and-away the most common getter past and present is Barium. Others
include Aluminum, magnesium, and any of several other materials or
alloys. Very high-temperature tubes use somewhat less-active getter-
metals as the temperature overcomes the otherwise limitations of the
metal and permits greater life and less volatility of the getter.

Typically the getter is flashed to the sides and top of the tube for
reasons already noted.

When the tube envelope fails, the getter very rapidly combines with
available oxygen and other elements in the air. Barium turns white.
Aluminum turns white, Magesium turns white. This designates an
envelope failure. The tube will be worthless even if the filament
survives by some miracle.

There are some gas-type tubes and rectifiers that use vaporized
mercury that have no 'getters'.

There is a lot more out there on this subject, but this should suffice
for now.

Peter Wieck
Wyncote, PA

 

[James Sweet]

I'd thought that was silvering to block stray electrons or something firing
out of the tops of the tubes. I'd thought this whitening process was some
sort of safety/failure indicator process as a visual indication of a
mechanically failed valve. Still learning even if an ancient technology

There's nothing that can fire out the tops of the tubes, a very few HV
rectifier tubes can emit trace amounts of x-rays, but the vast majority
of tubes operate at far too low a voltage for this. Electrons can't
travel in free air even if they weren't blocked.

 

[[email protected]]

There's nothing that can fire out the tops of the tubes, a very few HV
rectifier tubes can emit trace amounts of x-rays, but the vast majority
of tubes operate at far too low a voltage for this. Electrons can't
travel in free air even if they weren't blocked.

When Electrons travel in free air, they are called "ionizing
radiation" and cause all sorts of havoc. AKA Photoelectric-effect
Gamma Radiation.

Peter Wieck
Wyncote, PA