Call sign: K5jxh  
           I GOT GAS !  (In the electron vacuum tube)  Cures?

the electron flow path here , ionizes gas , there will be a problem. (2 voltmeters) The dots moving are electrons flowing. B+ ground is missing!

See Lethal warnings:
Tubes with gas can act dead, weak or make noise. (some tubes back in 1910 to 1920 were made gassy on purpose, I will skip those
1915 De Forest soft gassy tubes )
This page is about the good, the bad and the ugly.  ( a bad $2 tube we just replace it,  but a $100 and up TX tube, consider these facts below)
There are 3 classes of tubes,  (besides consumer and military / commerial)  Receiver RX tubes and Transmitter TX tubes, Rectifiers (Diodes)
The makers off all modern tubes, (RX /TX) when through huge efforts to expell, all gas  the day it was made. This fabulous once Secret RCA book covers all that magic,
See This book on history

Some tubes are gassy and normal like all old (now very rare) Mercury Rectifiers (Diodes) #86 or 886. Or the OA2 type tubes below. (My HeathKit radio has one)
Or some have harmless phosphorescence
Myths and facts:
The tube makes gas every day of its life,but is removed every day by the getter/keeper device. (keeper means keeps gas away after the tube is made and used or stored) (as stated in the RCA book)
Fact, all tubes have some guess now, for sure if you run the tube at crazy invalid over spec. plate voltages up to 100,000 volts it will glow.
Fact, some TX tubes can be repaired, (gas wise)
Fact , most RX tubes can not be gas  repaired,(beyond the keepers life span). (for sure any with indirectly heated cathodes as most are) (not owning an $200 induction heater and a ton of luck.)
Fact, all tubes have deeply hidden gas that should stay there,  unless the operator overheated and killed the keeper dead.
Over time the tubes Keeper, will remove any new gas, (minor) that you gain with age. ( When you see gas, that means the keeper is now dead !)
The Secret boot above tells you that there is a general why to know what type of cathode (K) you have: 
"Tubes below 600vdc are oxide covered cathodes, above 600 are Thoriated Tungsten, and above 3500volts pure tungsten." (paraphrased) Why do we care? Answer: the type  of cathode tells you if there are any gas repairs possible.

CAUSES of BAD GAS: (bad means , makes the tube fail as say an Amplifier or Oscillator or arcs over , makes plasma or the tube makes tons of noise from the gas)

  • Bad handling (physically) The tube is cracked at any location or manhandled in  way that cracks  the leads entry points into the vacuous envelope. (you let the air in, or the funny phrase,  the vacuum leaked out )
  • The owner dropped the tube. Causing the above.
  • The owner  over heated the tube,  made the plate glow red, or any other grids inside the tube overheated by running tube over spec.
  • The rumor that tubes gain gas after years in a box is mostly malarkey.   (not parked in a box they do not unless defective to start, like cracks are...) If in a box, the keeper would squelch any gas.
  • Old tubes are all gassy, not if they are top brand quality tubes; I have huge vast boxes full of tubes all gas free, 60  years old,  how do you explain that,? easy, they were made by RCA or other top brands (lots of those in 1960s up)
  • One last rare thing is that some very  old (1930's?) makers of some tubes had no (KOVAR MAGIC seal) at the lead wires.
  • EOL ,  most tubes at End Of Life and magcially the cathode is still alive but the keeper expires and now you have EOL GAS.
We see some cloned tube (KT88)  Sovtek tubes exhibit a  phosphorescence
We are told the plates with  Iron+cobalt+nickel will send dust to the glass and glows. (it would if there was copper dust inside)



Sovtek tubes

Sovtek tubes

Sovtek tubes


Modern means post WW2:
Most modern tubes made in the USA were made as hard a vacuum as possible, as it was pumped down using complex multistage pumps,  in the complex process seen in Refr. 1 to 5.
There were many top USA makers, here, at the time, 1940s to 1980s. When I say RCA here, I am talking about all top brands made in USA, I sure can not talk or say anything about imported tubes, at all.
RCA/Sylvania/Raytheon/GE/TungSol/ were top brands for decades.
or DuMont tubes that were re-labeled  from GE, Tung-Sol, Sylvania, and RCA.
Or Milatary JAN tubes of any kind (JAN = Joint Army-Navy) are fair game.
The lead-in wires should not leak,(but sure if you bent the pins or forced it the wrong way into a socket) (top brands, the pins don't leak, but there are many lesser brands today, clones that can fail here, cheap and bad)
Also know that most tube testers can not do GAS test correctly , in fact can't do the test properly up to voltage of say 600vdc spec.  (nor is this voltage safe to use on any by hand used , testers)
In the militray we were trained never trust any tube tester (unless it shows its dead) and to only test them in circuit, biases good, input signal at spec, and tube output ok a free of parasitic oscillations. (or not neutralized proper)

There are many regulator tubes not RF AMP.'s , that are full of gas,  Nitrogen , Neon, Mercury. top this list.
The OA2  tube in my rig glows all day long ,if not the  power supply voltage goes out of spec, too high, 150v is spec.
 (to the young mind thinks of these  as a vacuum powered zeners , sure an oxymoron that, but helps some folks born after these were invented or used every day) 
Did you know you can make a crude, voltage regulator from a NE-2 lamp?  90v reg?  You sure can for low current usages. (when I was young they sold the NE2 with sub letters for different voltages, but not today)
This cold cathode OA2 regulator, 100% normal normal.

TUBE Gas that is bad or  normal. (it depends on the tube and function)
This section is only for seeing how each common gas glows, the color of the gas is unique. (but do know the human Eye can not see  all visual effects can  fool anyone)
The gas can cause thermionic emission to be disturbed, if said gas is in the conduction zone or as some say or clearly ; the Transconductance zones.
More about gas,  The   OA2 tube glows pink/orange-ish above, and is normal, as it has purposeful gas , as are mercury vapor rectifier tubes and the Neon lamp just above shows.
Some  AMPLIFIER tubes, can glow blue near elements, due to  normal cobalt impurities, seen  glowing next to glass or mica spacers, the bad gas (purple) if seen between elements are bad.
The human eye has trouble seeing below green wavelength,  here are some colors of gases, and reasons.
(keeping in mind the human eye , peaks near a wavelength of 570 nm (green) and drops drastically approaching violet (near 450 nm) yes ,the human eye and/or even the brain does lie to us all the time... and many ways)
The human brain will lie to you to, it will see light flash and fill in the dots, and guess at cause (DNA reasons and learned) (there are great web sites covering this topic, on brain/eye fooling... and super interesting to learn)
The key to understanding gas glow, is not so much color, but its locations. (after all,  the 2 top  gases are  20% oxygen, and the more common nitrogen in an unknown mix)   (Air is mostly Nitrogen)
First learn some colors:
Below is the  WIKI set for gases, the center row is pure gas, only, If you work in a sign shop in VEGAS, one can mix gases and do crazy things.

Oxygen next, 20% of your air.(sea level) more wiki, photos,...
Oxygen O

And last Nitrogen, the major gas in our air.
the last color is Nitrogen , what our air we breath, is near 80% called N

Your tube will never leak air and glow like Ne? Not possible.

Examples of  all types of GAS. (seen)
Normal. red/orange filament glow on the left,  and oddly normal glow (we are told),are these are KT88 tubes, called phosphorescence.
The rumor is Iron+cobalt+nickel  plates contaminate the glass and is ok, as seen on Russian tubes.
The blue glow on the right photos is not in the center elements conduction path ( not good if seen there).   (keep in mind the camera can see some things, the eye can not)
Tube on the left looks good.
The KT88s do not have blue or purple gas in the conduction zones.  (only on the glass, is phosphorescence.) This is either lazy manufacturing (glass not cleaned) or the plates get hot and dust the gas and it glows)
The tuning Eye tube is good, and the Nixie tube is programmed to #4, both normal.

The funny green tube is and magic  eye, tube used for tuning a receiver.  (for tuning in the RX carrier signal dead center)
Normal EYE tube.  #6E5 (common)

 These are RECTIFIERS, used when then  need for huge current is paramount. (not wanting 4 times the number of tubes in parallel and costs there of)   Atomic element Hg.
 Today just about no one owns such tubes.  #86 or the like, I put this here so you can see the differenc of Hg glow and  Oxygen glowing colors.

Merc 823b in the middle. 

As you can see the Mercury blue gas is normal, makes aqua blue, that purple gas (nitrogen) may be bad, as owner said. (look for mercury droplets inside, do not shake this tube if mercury drops are seen there)
Always store MERC tubes standing up, if it came in them mail , make sling and sling out doors, away from any object,with pins facing outwards to get the mercury back to the bottom. (use great care doing so.)
I'd use a foot sock tied to a short rope. In such a way as it can not fly apart easy.  Do not spin it like mad, just gently. Swing over your head to it can't hit the ground by accident. Many new tubes like this have instructions to do that.
Best is bet , is to up grade the tube to solid state diode sockets, sold all over the place. (ham stores, relic)

Next  UP, some ABNORMAL: Bad gas tubes, for sure.!
A good tube has a hard vacuum.  (special pumps did that and with final Getter activation gets the rest)

I have 1966 TX tubes (Vast collection of tubes, all from 1960-70s era and have zero gas, so  I think AGE is not a factor, abuse is.. (dropped, twisted badly and run over spec, overheating them does this)
Let me explain, abuse, burned up the grid or plate?, or ran  the 6v  filament at 10v or someone dropped the tube, or ran the Transmitter with all vents blocked? Abuse has many forms.(or bad luck too)
Tubes can be overstressed and if this done, deeply contained (hidden before) occluded gases inside will work there way out, and if the tube as  still functional getter , this gas may be removed, if not , then not.
No metal in any tube is gas free ever, so if you do something wrong (bias it wrong) gas will be deployed.  (think of this as squeezing a sponge and water comes out, the cure is end the squeeze)
The bottom left is gas.  From @K8JI ham site

The above right tube, is a crazy torture test, ( owner ran huge voltages , 50kv? and xray emissions (crazy acts here and very dangerous !)) NEVER DO this. ever... Do not make Xrays , on purpose. (over 30kv this can happen)
If you accellerate any electrons above 30kV levels, they can hit metals inside and knock off xrays (atomic electrons are knocked loose).(old  70s TV sets had a  regulator to prevent this a 6BK4 to prevent XRAY emmision !)
Do not attempt to make XRAYS ever. ok? My Transmittters never go above 2500vdc.

Tesla would be proud?  after all his demostration of RF wireless lighting , works just like this.
Next is my SB401 Transmitter. My playing around safely,  hands in pocket, but one hand on the key button cable at a distance,  Shot with telephoto lens, far back.

Here is a tube, of sorts, it's called  a florescent light bulb (as seen at Home Depot) with ARGON gas inside it. (this lamp actually has a dead internal ballast circuit and fails to work at all in normal usage !)
The bulb is not even connected to wires here, but is glowing by my Transmitter (EMF) RF field  of 100 WATTS , Keyed, transmitter, here. CW. 7 meters. (new PA tubes day, and upgraded)

In 1893 Mr. Tesla did this at the world fair.
He wanted to show Fluorescent lamps work and run cooler, and can be excited with RF fields. I'm doing this below , just like he did. "wireless lighting"  (We had no FCC then to slap him down! for illegal emissions ! )
He also proved AC power distribution worked long distances, unlike DC, (then) << His biggest invention, that, AC /transformers and motors that run brush free.
Doing this helps young folks learn to respect HV and RF.  Seeing it helps.  

In the olden days we called this the DOG house, and same in all TVs sets then (29kv there with Color TV the 6BK4 tube would fail and you get Xrays over 30kv , a real danger there on huge Color TV's)
The old TVs even had leaded glass in the TUBE to lower Xrays if the 6BK4 fails. (a super common event then, by the way....)
In the Military, all such places (dog houses) have interlocks , that shut down the gear , at first ingress attempts , the dog house label states (Lethal voltages inside)
Pure RF ionization here . From 6146 twin TX tubes (fresh and new) inside.   By the way this lamp is dead, using in a real light socket, and is why I picked it, for this test...
This is normal, this dog house can not ever do Xrays.( I mention it only for folks messing around with 30kV+ tubes or playing with wrong power supplies of the like kind)
Keep the lid on unless doing this test. Do it for like 5 seconds only. (just to see if there is a huge HF field here)
   HANDs IN POCKETs HERE < DO NOT TOUCH THIS, 800 to 2700v or more..

HUGE TX tubes, I can only dream about, ($3000 transmitters are way to expensive, no?)
These are a beauty no?
This guy used a Telephoto zoom lens, and did not get this close, after all you could sneeze and die , in 1 second. (learn this, NO SECOND CHANCES HERE !) A ticket to Saint Peter.
I don't see a Getter here.
One more normal glow, for 3-500 Final PA tubes, no bad gas. (pure filament glow)

Some tubes have elements (see periodic charts) added inside , called Calcium oxide, CAO or Barium oxide (BaO) or Cobalt (from cathode area migrated.. else where.), that can land on the inside glass. and glow this is normal.
Some tubes (transmitter mostly) have radio active or toxic elements, and it be smart not to ever bash tubes and break the glass for fun. (look up your tube spec before, just pitching it to the trash) it may BE HAZMAT.
There are oxides inside that are mostly harmless if not breathed or eaten or allowed ever to land in the eyes.
The real danger is the Thorium in the filaments.(it is radio active, as is all 27 isotope it decays to... says madam Currie) If you see no indirect cathodes inside, thorium is present.(thoriated)
Always read your data sheet on the tube first, and never put  it in the trash if it is a radio active tube.
On some tubes the gettering gases can silver coat or black coat a internal parts and then glow.(harmlessly)
The word GETTER is not today word, 2018 it was used from 1910 to 1980s, and ended in the consumer world then. (not in commerial TX world)
A getter eats gas. (think of PAC-MAN for gas) It is seen as a channled ring or  tank (cylinder) shapped object inside. (when just made new, it is flashed (induction heated) then later it eats gas fo the life of the tube (Thoria' goes dead?)
Watch grid current , carefully, and not allowed to rise out of spec. or you may gain gas in the tube. Never operate any PA transmitter out specl or fail to  monitor grid currents.
Do not make the grid glow red, do not exceed max plate Pd , Power dissipation spec.
Some tubes can glow the plate (anode) center surface normally, a very low weak dark red. (do not exceed data sheet specs) but only on the most rare tubes that do this, like above.

These fact alone make  glowing diagnosis very difficult,  but location ishe key he tre, on the glass it's harmless glowing or is also seen on mica wafer spacer ring mounts.
You don't want the glow, blue/purple in the cathode to plate region, the , the "transconductance zone"  Cathode >>> grids>>>>Plate zones. (ever)
The electron cloud and beam moves in the conduction zone (cathode to anode area, grid in middle)but the same cloud of electrons, (some) can wander  about, finding odd things to excite, as seen below, clearly.
Some tubes have mica spacers, that can glow to, harmlessly.

More normal tubes., the right one is scary, but normal.
 I think they call this phosphorescence due to some excess  material on the inside of the glass.  (ignore this)
If you read the books by Kohl,  (pg556) here:
You see most of these odd effects below are just the maker cutting corners during manufacturing, ( the hot hydrogen cleaning steps, skipped ? for sure the glass cleaning was... in many cases like this...)
note the glass hugging  glow here.

DOA, Dead on Arrival.

For SURE BAD Tubes are next,  note the "classic nitrogen purple glow" and locations.
845 on right bad gas,  note the color !
The getter pie plate pair seen is on top left tube and had, a silver black shinny coating once new, like the middle center tube has here,  but after the glass cracks the nice silver oxidizes and turns white, proving huge gas leak.
 Don't run this tube, it yells at you.

 Do not break up tubes and play with them , many have radio-active doping inside. (no nukes?)
Sb-1000 boom.boom
Boom the main HV fuse blows. (both O2 and N2 seen here!) 2500 volts here, at least.
Those 3-500Z are cute no?  For many hams it's love at first site, For many hams , if you don't over-load the tube?, it will last 50,000 hours,  (this is a life time for any ham)
Overload means, out of data sheet spec. (It does not mean running normal and full power)

Keep the Faraday cage around the tube, do not run the rig lacking full cages,  the dog house as we said long ago.  (the RF fields at HF frequencies can damage your eyes, this close) see that wire mesh, it's not there for kicks.
There is  wiki on Eye damage from close in (near field) RF electromagnet, fields this strong, DON'T DO IT.  (if you must stand far far back)

This is both a SHOCK hazard exists and the flooding of your shack full of RF fields.  Do this only during actual trouble shooting..(meter tests, etc) testing .
 Power off, connect meter, power up , stand back with hands in pockets, and key up. (make sure the RIG has  real ground , chassis to the wall AC safety ground pin) (or get rid of Heath kits, 2 wire power cables as I did)

The best gas page of all  is here below...

Nothing can be so dramatic as a RF linear amp with gassy tubes,  most IONIZE then ARC over, then blow a fuse, if lucky the fuse saves the day, if not, more parts blow up, with that surge.
These tubes are why more exciting than any FET. 

Vacuum:    (seem custom hand made radio tubes here)
The #1 enemy of tubes is that vacuum inside, nature wants to be invaded every day of its life. As per Aristotle so clearly stated in the 300 BC era. ("nature  abhors a vacuum")< he meant only on earth, not on the moon.
One can say nature favors a vacuum, like in DEEP SPACE, but  he was not a space man then nor were his friends, so he left off the word on earth.  (he was "grounded" then ,no wings and that word I will never try to explain)
I had a wild idea, to make a Tube radio sitting on the moon  and then remove all the glass around said tube  (carefully just above the glass base ring, but will the radio work? or with the electrons get lost? and fly to mars.?
 I think it will work.  (after first setting up an AM station first on the moon, solid state ? near by  , I think earth is too far away... to pickup KGO, a super station)
We can rename the radio the A.M.  "space tube radio". (no glass) and or the zero state radio, runs only in a hard vacuum of space.
 I think cosmic rays and solar winds would cause lots of noise, here, but who cares?, it's AM and if already chock full of noise. (end digression)

The tubes when over stressed, can create there own gas, from the metals inside.(stress caused by damage in the metals inside (grids or plate) "overload stress, is real ,avoid it"

In the factory they pull a vacuum on that (see #9) below seen nipple, it was a small glass pipe at day 1, and they pull (means pump) a good vacuum.  Usually a dual stage pump is used. (of some kind)
Then a next stage pump kicks in (a Torr grade  pump 110−3 Torr) and pulls it down harder, and last it is melted and sealed, there are 7 levels of vacuum seen here, and tubes run at high vacuum. 
The vacuum on the moon is 10−11 Torr.
AT the #9 nipple below.
The part 10, the Getter is the flashed, using and induction probe of some kind (electro-magnetic RF induction,or direct wired)    See gas cures here.
 The Getter device  #10 below has the purpose of removing minor gas build up. (it has a dual function, day 1 heated up at the factory and for life of the tube) If the getter device depletes,in time, the tube may be doomed.
The RCA secret date codes are here.                see this next LINK to for the Free tube manuals in PDF form.
We gringo's call them just de Forest  "tubes" , in Europe they are called Valves or  Fleming Valves but , RCA was a giant then, and we now have great free books, on there whole product line.
Hey bloke, "Me  Fleming is a bit dodgy".(UK slang)
The correct name is Receiving tubes or Transmitter tubes, here.  (one of the first major tube makers here Pre-RCA it was Western Electric)
 (Ignoring soft tubes used by Ma Bell telephone line repeater, etc) RF first major usage for the new Radio age. See a 102 tube here, (and 101 series)
Hams call them Hollow state technology, there is nothing wrong with this technology at all, it works great !, the heaters are the only problem, as they waste huge power. (well   not really  huge, what 30w? in my receiver)
I have owned vacuum tube pocket radios (Emerson) that used very tiny tubes,  it had super tiny low voltage low current tubes (triodes,etc) inside, very much like the 2nd device  in photo  3 below. (or like this)
There is also a newer small tube called  RCA Nuvistor.
The names of batteries, long ago A, B and C and D were for portable Tube radios, and for sure in towns with no POWER GRID .  (to hear FDR's fire side chats?)
Photo 1:                                                                                                                                                           Photo 2:                                                                                                                                                           

Photo 3:
The left device is a  NE2 lamp and is not a TUBE,  with 2 elements.
I worked on Military  URC-9. using a pile of 7/9 pin tubes. (is really designed for aircraft 400hz 120vac power, is much lighter weight.)
Ask me how fun it was to get the fans to work in this 1750 channel transceiver. (it had the magic fan gear box (step ups RPM),with no actual gears, can talk  for days on topic (endless hate and no new spare parts..))
The first ever crystal sensitizers (VFO) ever seen by me,  in this gear,(rig?) by me 1970, in the '9s as we called them then, were MIL contractor only black boxes, no documents for the insides,. (a trade secret then)
My buddy ET had , just deep 6th the old UHF TED/RED gear.(AN/URR-xx  (13,27,? ) and older sister the   AN/URT-7 VHF, the URC-9s,  displaced this old gear.
That was 225 to 399 ,mc, motor cycles we said to be funny.[mHz] (UHF) line of site comm. plus Crypto. KY.

RADAR:   (large powerful tubes are not new ideas)

Here  is the transmitter tube used to allow detection (main bang pulses ,it's called ) of echoes from the ACTUAL  Japanese ZERO attack at Pearl Harbor, 12-7-1941
 ( omg sarge, their not squawking..."I.F.F". codes of the day.... so they guessed , Hark B17's it must be, but  had no IFF then,,,, opps, .... bad news....)
Amazing now the RADAR Operators had no list of scheduled flights out or incoming that day. ( Their  boss was , Tim Conway?  Ensign Charles Parker seems so to me...)
Triode water cooled, (pure water, via special systems) WL530  Typical pulse output 75 kW per tube. 8kw plate dissipation.  (range 200 mile with aircraft up high)

Gas by accident (over driving a tube):
The tubes can be over stressed, can create there own gas, from the metals inside.(stress caused by damage in the metals inside (grids or plate) during abuse, or bad grid bias.

The below tube is from 1957? A TX tube. (In my private museum) Very Cool logo , no?
Note the Getter/keeper drum top center.! I think this drum is for life of tube gas eating duties.
That drum there, is tube in a tube device, the drum is glass and the keeper deposits are clearly seen on the glass of it and not a drop on the acutual tube, now that is cute.

Topic GETTERS /KEEPERS and the LIKE !  "Gas eaters" types and possible recovery actions."
What we can do and can not do about bad gas. (And how Gettering can help you)
This getter, is ring or tab or drum  full of Magnesium/Barium/Aluminum (very common elements used)  The Barium (Ba.) part is the KEEPER function.
The maker of the tube knows the getter flashed in the factory day1, but wants to know also that the Barium is deployed. (I guess done  visually this QA step, I don't know for sure how ,but the book talks about it !)

See them use induction heating to run the getter a 2nd time.

Once the getter is flashed day1 in the factory then the KEEPER does the duty of keeping the newer gas at bay.
Are there ways to remove gas now? (the tube is bad, by the way)
Most times not. (building $200 induction heater to save a $2 RX tube, does that make sense?) Most RX tubes can not be saved, by any means.
The getter/keeper is now dead, if the tube has gas. (for sure seen gas effects in the transconductance zone)
The only reason I can think of trying to cure gas discovered, is in a very expensive transmitter TX tubes.
Many TX  tubes use PLATE Graphite to fight GAS too. Some huge TX tube have no getters at all, (seen or found in any visual  way) but the getter is there in the PLATE.

The getter was flashed at the red hot pump down day one, then as you use the tube, the keeper  may still be awake (material) and still eats gas that is normal.
The getter my just be a tank/drum (seen above) or ring of barium alloy.

But many TX tubes have this...
There are many forms of Graphite all made from carbon at very high pressures.  This Graphite also fights GAS.
Atomic grade Graphite A9"Hex" is what is best. formed at 2500deg C.  or more. (Kohl quotes below)
The more carbon in the steel the more gas it can  hold (that we don't want).
 There are many methods to prevent to prevent gas from forming as you  use the tube, to stop deep Occluded gases. (Nickel metal plates win  this 1st step)
Some maker use the far better plate material Nickel.  (or Nickel  plated steel and the Graphite over that)  The only advantage of steel is lower cost.
See comments under  high powered tubes here.. (page 161 Kohl)
The very hot plate in operation can burn up gases, in this Graphite (elecroGraphite is the real name)
Electro Graphite is used in large power and most Glass transmitter tubes, one of a tubes prime features . This material is SUPER PURE.  Like my 572's seen here.
  • Lets the gases below it in the base metal, pass through and get purged easy. pg 166
  • High radiance efficiency, lowers its temperatures, page 157
  • 20 fold increase in thermal conductivity. (and increases peak anode power dissipation Pd)
  • Graphite has a very low secondary electron emission spec.
  • Low thermal expansion rate prevents overheating damages, like spalling ,cracking from thermal shock.
  • It as the ability to burn off tiny present gases. While hot.
The Graphite allows gases deep in the Anode to release easy when heated during production. (if the correct metal is used, below there will be little gas there and what remains, is gone at 1000C
The last step in tube processing is the Getter flash, after the tiny exhaust tubes melted closed;.(best way, but done before the  melt on cheaper tubes.
On sealex machines the cheap RX  Tube is made in 1 minute flat. (say a small 7 pin receiver tube) so when the keeper dies the tube goes gassy, no cure.
The Keeper (part of Getter) on most tubes eats gases for the life of the tube. { see pages 70 in this book from RCA, 1938 }
Some folks try to reactivate the Getter/Keeper. (mostly tried with no results )
If the tube owner overheated the tube and then wrecked the keeper, that will be the end of life for this tube.
The Getter inside the tube is Barium /aluminum   (50:50 b.wt) and eats many gases,  (or Barium/Magnesium/Alum)
Old parked tubes found in the attic, may have gas, some may have been parked full of gas, but most tubes to not grow gas parked, its heat that cause gas to migrate.(mostly and at full opererating temperatures or more)
The Keeper might wake up and clean and old tube, that has gas by just running the filiments.

The getter warms (by heater action) to over 200C and eats more gas,  see this book on topic (Refr. #9 W.KohL)
Gases it can eat are: ( those are atomic symbols for gas and in there natural state)

Photo below is a  cut and paste from the free KohL book.
The 2 big ones are Oxygen O2 and Nitrogen N2.  (the 2 primes in the air you breath and what the tube was exposed to , before it was born (evacuated and flashed)

If you wrongly overheat a tube a TUBE or its metal elements so badly that you exceeded the manufactures spec.'s, you many release , gases from far deeper in the metals.  So don't.  (even the grid can be overheated)
Again, no tube made is gas free, all tubes have for example oxygen buried super deep in that metals, inside, steel/nickel .  Steel holds more and leaks out the most easy.
That no tube maker on earth could remove (Deep Gas), even with 30min. bake outs at 1000 C and pumps. (nor cured at the FLASH of the GETTER)
We have vast NOS, tubes near or over 50 years old now, parked on the shelf and are gas free.  (RCA HAD GREAT QUALITY CONTROLS)
If you find gas now the tubes Keeper is dead,  there is only one hope now. (mostly) and burn the tube hot with no HV on the PLATE, if that fails try running the TUBE at low full power, (1/2 plate voltage)
I have one of the  best tube testers ever made to check gas but is near useless for that, as I would expect.

Some tubes if you run the heaters too hot it makes it worse (RX mostly), others it helps TX tube.   150% or 220% over voltage, but the maker has chart for how long, and if ignored this will damage the tube.
I'd avoid playing with excess Filiment heater voltages.
In fact, in the factory this is done, with a very specail set of rules, voltage and time,  and is a trade secret. But covered here. This fabulous once Secret RCA book covers all that magic,
TX tubes are very different from any RX tube.  (It is wise to study your exact tube first, how it is made and methods of recovery)
Some TX TUBES that are metal cased can be HV pulse refurbished.  EIMAC has books on this.

I think just running the TX tube with no HV , and only on filiament power, is the best way to eat up TX tube gas...  (run it on heaters only for say 24hours, I know a guy that did for a week and it cured it)
(study how the  TX tubes  filiament Thorium (Th) can be reactivated.,  it starts at 100% day one and when at 80% the tube is dead)  The Thorium is only about 2% and 98% Tungsten. That 2% Th.  reduces by 20% ends the tubes life.
Some tubes like mine like to be run at reduced power to self clean of gas (Graphite plate rule 572 tubes and like kinds) Do not fail to try this, It can work. (Even run the HV 1/2 plate voltage if you can safely)
Learn what your specific tube needs.
The most wild idea of all is one guy built a $200+ home made DIY, Induction heater, and reheated the Getter/keeper ring/tab or barrel. Wow ! That be fun to watch from say 25 feet away.

3-500, example:  (a 4000vdc spec, max anode rated tube, from Eimac.)   THE PA BLUES (RF power amp)
If this tube goes gassy, the meter on the PA will blow up, for sure set to grid current, so why did you fail to put back to back, for anode to grid shorts.(or gas caused) each diode below is 2 series, 1N4001, 4 total.
This is classic fix below, if you open any modern instrment today with this meter inside (coil
D'Arsonval) there are 2 diodes there, or 4, and saves blowing up a $25 more most costly very hard to buy meter, for 12 whole CENTS.
SB220 , ( Have  this done to all 4 of my Heathkit rigs)   
or like this wise choice. (max voltage in is 1.2vdc ( set a clamp just above that)
GAS here starts with small glow then fully ionizes the gas, the turns to a plasma and now ARC,  if this is not clear, avoid tubes. No your 0.5v biased DMM set to Ohms will never find or ionize gas ever.
That is what happens in the blink of an eye.
The only time a DMM works here is for a dead short ( say grid collapsed, (as in bent) Yes, hard zero ohm shorts can happen of the tube was overheated. (it only has 3 elements) 
You can put heater resistor protectors here too.

Some TX tubes do not have actual obvious Getters,  like this tube 3-500 (upto 4kv) , that I think the getter/keeper is built into the plate (anode) It does have a Graphite Plate and Zirconium for gettering, or keeping !
Some say:
To getter this tube, jig it up as "Plate at 500 volts,Grid at +60 volts,Plate current will be about 700 mA,Grid current will be about 200 mA."Then leave it for a day, and then do hiPot tests. (testing for acring is  a whole other topic)
This answer above  is actually silly (in quotes), the correct answer is start low on plate voltage and raise it in the jig to see if it arcs below 1kv, after all spec is 4kv, so if it ARCs real low that is HUGE GAS not just tiny.  (and hopeless)
We don't know what is bad (extent) nor how bad, bad comes on many forms, and at 3500vdc, please take stock of that, ok,?  Do not just dump found ebay tube  in your right turn it on. (use a Variac first or build a jig that is safe)
Better yet a jig with variac in the AC line. (run full AC on filiaments and the variac for the PLATE supply)
This gassy tube will blow up your rig (PA) so a jig need to be made, and safely.

Adjust the grid drive and/or plate voltage so the plate glows a dull red. The tube must have forced air fan blowing at all times.    Get the Anode hot, and the Zirconium/Graphite then wakes up and does the job.

At the factory flash step, putting a mirror like coat of alum/magnesium on the glass envelope will cause blockage of infraRed heat  from plate to the outside of the tube, a very bad idea that. (on super hot TX tubes)
from the RCA books (many) The chart proves how hard it is to find out what you have.

The Atomic chart above you can sen the natural elements end at 94, (made by GOD or the BIG Bang (take your pick)  Many elements are man made, or the results  of atomic bombs or atomic reactors (OF MANY KINDS).
Half joking if you own a powerful neutron gun , you to can make things, nobody in thier right mind wants to touch or get near. (LOL)
This chart is off  Wiki, (just to show how good it is)

Last are the toxic natures of vacuum tubes, if at all. (hint , most are not !)  Vacuum Tubes or Edison(De Forest )tubes are what we call Thermionic devices. (it gets hot and makes lots of electrons)

Most tubes today that are danger to anyone are not the receiving RX tubes, but are the transmitter TX tubes that only uncle Fred the HAM fools around with. (me)
The only serious threats are simple , over 600 volt tubes may have the element seen above.Thorium (Th 90)locked in side the  harmlessTungsten (74) metal called the Heater filament.
There are no radio active isotopes in these tube, nor any elements above atomic weight 82 lead. (besides Thorium)  Bismuth 83 is only extremely weakly radio active. (and is older that earth element (big bang old)
As a  general rule in life stay way from things above atomic weight 83. (Tell Madam Curie that then)...
Most of the oxides in the tube are safe, but do not eat , or breath , any broken tubes dust ,
Do not break , open vacuum tubes on purpose.
For sure on transmitter tubes,  do not file or grind any cathode heater wires, ok?
The other common element in most tubes, that sounds odd or scary, is Barium Ba.{56 }oxide (BaO)  and is safe, but do keep it out of your mouth and your eyes.  But is no more unsafe that driving a dirt road. (I bet less)!
Barium is rich in electrons and is used on most RX tubes,on the cathode and Getter. (all other metals seen here are safe and all oxides are irritants in this tiny amount) BaO is also seen in the glaze of your dinner plate.
The amount of this oxide inside is miniscule and  evaporates  fast in free  air.  The thinkness is  very thin, if  they put too much on the cathode the tube becomes unstable, so is thin by DESIGN. and far safer.(with a broken tube)
BaO is listed as an irritant,  keep it out of eyes, and mouth and skin but never inject it (why would we? and is lethal that way)
"Exposure to air allows the oxide to form hydrates which can cause flaking of the barium oxide. " (the water H2O in the air does that fast)
"Barium hydroxide presents the same hazards as other strong bases and as other water-soluble barium compounds: it is corrosive and toxic."  (Ba(OH)2) but not listed as extreme hazad.

One very useful tool today and with the modern internet, is looking up anything in the wiki or MSDS sheets on elements. " keep in mind even too much water consumed in a short time can kill you"
All things on earth are toxic,at some ingestion levels or odd methods (injections? wow !),  the key to all this is the AMOUNT, and the above it super tiny amounts.

After reading this TUBE STUFF,  you rob a  bank and buy this, right?  (but my PA is $100 + pocket change upgades) 
I do hope your tube can be recovered, easy, or your old TX tube. (the RX tube  I think are a hopeless cases, due to the rules on indirect cathodes)
That Cathode in the RX tube has 1 to 3 oxide layers (very thin), all of which turn to carbonate if air leaks in!  (and wrecks that oxide totally and tube is dead, but heating the tube (after leak fixed, haha) to 1190K (kelvin) or 2170F.
Nobody can do that to such a tube after the fact of air leaks. (even if the leak  is cured and the Getter/Keeper still reflashed,  the Cathode will still be dead as a door nail)
The Sealex machine made by RCA for their plant in NJ,  seals tubes real fast. (in minutes)

Never voltage stress the filament  any RX tube.  (the cathode will be wrecked)

References (top)  (and odd hints)
  1. All RCA books in print, (most are now free to read in pdf form, Google them , for sure RCA "Electron Tube design" 1962 edition" wow ! See page 508 for very good degassing methods done.(in order)
  2. Best "Saga of the Vacuum tube, by Gerald F.J.Tyne (he took me back to 1883 by T. Edison "Electric effect tube" page 33, 1994 last publish date by Tyne.  1905 to 1930s best history ever, one book.
  3. But for sure the most interest to me , is this mans books, now from long ago. 1920 MA. PhD.Van Der Bijl , I had no idea such great information existed in 1920, in this 10 chapter book 408 pages. 
  4. The pages to read above, by Bijl cover gases, and cures. (some) sections 38-40 page 102.
  5. Materials and techniques for electron tubes by Walter H. Kohl. 1960  (a very good book)  628+ pages,  Page 556 tells you  how  occluded  gasses are  removed  
  6. Eimac (now Varian Assoc) in "Care and Feeding of Power Grid Tubes"  Seen at CPI
  7. The commercial over 10,000v tubes can be ZAP conditioned, if gassy,  using this method. (just food for thought , on a gassy tube, one might try this, carefully  , do not exceed 5000vdc. or your max tube spec. )
  8. Gettering,  658 pages,   See, Walter KohL book (public domain & Free) . HANDBOOK OF MATERIALS AND TECHNIQUES FOR VACUUM DEVICES (covers GLASS and Seals , and lots more)
  9.  See this fabulous once Secret RCA book here, a gold mine of facts  here. See pages 10 to 15 to see how tricky the cathode Oxides are ACTIVATED.
  10. Getter page,  dense
  11. The getter flasher magic device (inductive heating){video}
  12. Many TX tubes use Pyrex glass (Corning brand) and only can use 1 type of glass seal, seen here.( this shows how tricky this sealing up business can be)

This is off topic and off track, but is a fun flashback in time.
Fun history here: 

They lie that JAPAN invented small radios is an  endless myth, they only made them so much cheaper then ! (labor cheaper), they put OUR guys and many others out of business. and . (or like this)
I even sold Regancy TR1 radios long ago.  (Japan never did invent this radio, ever, they cloned it)

Noge below the ONE VOLT TUBES, and the A and the real B batteries,  (Later A became AA and AAA) later, we had , tubes + final amp transistor radios called Hybrids (at that time)
Some old radios had 3 batteries,  Filament, Bias, and Plate supplies (yes  a small horror then) The below radios use 75mA of filament current alone, 10 times what the transistor radio needs to operate.
1954 (and very obolete even then; see TR1)
1958 USA !  (sorry not from JAPAN)

Check out the mid 1950s radios by  Zenith Royal 500,  General Electric 678, Arvin 9577.
Japan began transistor radio shipments  in 1956, well after the USA made and sold tiny pocket radios. (of VAST KINDS).

Now an odd duck radio:

The radios even evolved from tubes to transistors full.  They were sold as Hybrids then.  (I sold them in  Electronics store then , that  and HighFi, Ham and parts)
The tubes like this kept shrinking every year, the Russians developed beyond the USA here,  now tiny as seen  below, and on  the MIG Aircraft some had radios using even more small secret tubes... below are 8.5mm diameter tubes.
No, JAPAN did not Invent pocket radios, (but the joke is funny in the movie "1941" As "SLIM" boards a JAP SUB. LMAO.

The first USA Fully transistor radio is below, TR1,  , in fact we called them Transistors , "got your Transistor?", lets go to the beach. (like today Got your CELL?) This makes the Nugget above look like junk.
The advantages of this 1954 TF1 Radio below is not size !, NO, but now it has only 1 battery and lasts 10x time longer.(wiki images)  The radios above eats up that filament battery super fast. (1hour?)
The above radios use a total 75mA of filament current alone, 10 times what the transistor radio needs.  (oddly todays batteries can do 10 hours easy with 75mA load but not back in 1954)  For sure  way less battery costs !

See how Texas Instruments help make the above possible. (100,000 of the pocket radios were sold in a year._

Some of these early and more cheap radios had no speaker, only a single ear bud, worse still it was a crystal powered bud, (not an electromagnet transducer as all are now) and very very hard to find one good or used/new, and horrible sound.
See some here.

Now my 1955 hybrid Emerson 838 , 3 tubes, and 2 pushpull transistors to drive the speaker.    $44 cost then, and would be $410 now. (ouch) Now you see why Japan sold these later for $4.
This radio was Called then, a Hybrid.  (this made a better and stronger sound) but the "B" battery 45v was very expensive. (owned by me for 10  years)

the guts of the above, see the 2 transisitors top left.  (relic city here,,,,super old 1955)
They called it transistor but was not, it was 1/4th transistor in truth. 

LETAL voltages inside, Warning !!!
Before fooling around tubes,
know that many have voltages present that are LETHAL, (means no second chance of surviving)  
That means power cords pulled, and discharge all CAP.'s first using the stick below grounded to the chassis.
Use a chicken stick grounding tool after unplugging the Radio(et cetra) and waiting 5min. first)
Some Tubes have 2,700 volts on the PLATE (anode)  Mine does.... and more... on full gallon transmitter PA's
Never ASS-U-ME that any bleeder resistor in any gear/radio or transmitter still works. ( or death can happen so very easy) , In fac, ASSUME its bad or missing.
Nor assume the TX plate volt meter works at all on that TX or Transciever or PA.  panel .

version-8.  9-30-2016  (revised 4-21-2017 last time 6/2018)