Baking tubes ?

[arr2036]

One of the things with NOS stock especially the stuff on eBay is that the vacuum in the tube can become contaminated this causes all kinds of problems such as increasing background hiss etc and will of course shorten the life of the tube (the heating element anyway).

Fortunately there is a solution , if you look at the top of most tubes you will see a silver cap , this is a metal "getter" (usually barium) put into the tubes before they are sealed , the purpose of which is to draw the last the oxygen out of the tube in an attempt to create a complete vacuum .

With NOS the “getter” stops working properly and it is necessary to reactivate it, which involves preheating a household oven to 170c and baking the tube for 6 hours.
You can tell when this process has been performed because the white printing on the outside of the tube will have browned due to the heat.

Apparently most commercial tube retailers should do this anyway, just those odd job lots on eBay and such which may need the process performing.

 

[Glod]

Interesting. I don’t know if I would dare trying it. Have you?

Can the socket really take that kind of heat, for such a long time? Doesn’t the base glue get brittle and loose grip of the bottle afterwards? My 6SN7´s do get hot under operation, but I doubt they reach 170C

BTW. Can you please specify how the NOS get contaminated inside?

There is also the story about the 12SX7GT, which allegedly was driven hard for 500hrs before boxed and shipped. Could that be for similar reasons?

 

[arr2036]

Yeh i thought so too, i have some spare signal tubes lying around so I’ll give one of those ago and see how it turns out.

The tubes get contaminated because the seal around the base of the tube is never perfect and over very long periods of time it will let small amounts of atmosphere into the tube.

This advice was given to me by someone who used to design and build tube amps, based on studio design schematics published in journals

.

I was posting the info in case anyone had some problem NOS tubes which they didn’t mind risking ..

As for the 12SX7GTs i know some of the higher power tubes needed to get to quite a high temperature for the getter to work properly, they used zirconium of titanium "getters" .

Could also be to break them in , i know during the burn in period the amount of electrons being boiled off the cathode fluctuates .

 

[braillediver]

“Yeh i thought so too, i have some spare signal tubes lying around so I’ll give one of those ago and see how it turns out.”

You’ve got to be kidding me? You make some whacky statement like you did about baking NOS tubes as though it’s some “Known Fact” then later in another post you state that you’ve never tried it.


You can’t re-activate the getter. Initially it’s electrically “Flashed” which activates the barium. As the barium absorbs gasses it turns white and flakey, it can’t be reactivated by some external process. The material can only absorb so much excess gas before it gives out.

The getter stops working when it turns white and flakey. The idea that NOS tubes have vacuum problems simply from being old is a stretch of the imagination.


I for one won’t be putting any tubes in the oven at 170 deg C.

Nothing personal but I’m not sure your suggestion is a good idea and you don’t base your statements on any published information. So yeah everything I’ve ever heard or read about vacuum tubes in the last 4 years basically contradicts what you’ve said. When the tube loses a vacuum it’s toast.


“One of the things with NOS stock especially the stuff on eBay is that the vacuum in the tube can become contaminated this causes all kinds of problems such as increasing background hiss etc and will of course shorten the life of the tube (the heating element anyway).”

I just think this statement is…….. I got my hip waders on buddy, it’s getting deep out there.



Mitch aka born again skeptic.

 

[Len]

Baking tubes seem to work (sometimes) if you're trying to rid excess gas. I've baked a few pentodes that had excessive blue glow to them, and after about 6 hours in the oven at about 325 F, the blue glow went away. The bakelite bases and glue also seemed to hold up without problems.

FWIW, I think I've only done this a handful of times for experimentation's sake. I do not know how long a baked tube will retain a good vacuum. I would presume that if it's a problematic tube, the seal is irrepairably bad and it's only a matter of time before the vacuum goes bad again.

On a similiar note, there are processes defined to rejuvenate tube emissions by running the plate voltage 2-3x normal. However, it only works on tungsten thoriated plates. If you try it on any oxide-coated plate (and most tubes fall into this category), you will damage the tube. Believe me .... I've tried

 

[sacd lover]

The tubes get contaminated because the seal around the base of the tube is never perfect and over very long periods of time it will let small amounts of atmosphere into the tube.

This advice was given to me by someone who used to design and build tube amps, based on studio design schematics published in journals

.





You are describing a gassy tube. I would think once you fire up the tube the getter would be activated then. Once a tube has an air leak the leak is there. Unless the baking caused the base to reseal itself I dont see how this would work.

 

[arr2036]

Quote:

Originally Posted by braillediver “Yeh i thought so too, i have some spare signal tubes lying around so I’ll give one of those ago and see how it turns out.” “One of the things with NOS stock especially the stuff on eBay is that the vacuum in the tube can become contaminated this causes all kinds of problems such as increasing background hiss etc and will of course shorten the life of the tube (the heating element anyway).” I just think this statement is…….. I got my hip waders on buddy, it’s getting deep out there. Mitch aka born again skeptic.

There was the oh so subtle "?" in the post title but i guess i should have been more clear .

Surely oxygen in the tube would shorten the life of the heating element the same way oxygen in a light bulb shortens the life of the element , remember they are basically the same , as mr Edissons first diode was basically just a lightbulb with an anode .

Electrons hitting other particles inside the tube would cause them to be deflected going off at odd angles and possibly striking the anode?

Also were not talking about huge leaks here were talking about stuff that has happened over the 50 years + the tube could have been sitting in storage , if you think about all the temperature changes that will have happened over that time with the base and the tube expanding and contracting at slightly different rates.

I'm trying to figure out whether there is any truth to this baking thing as well as i too have read completely contradictory information.

 

[Len]

In nearly all vacuum tubes we use, the getter is activated only once - at the end of the vacuum seal process. The getter is quickly heated to a very high temperature (well in excess of >200 C) until the alkanine earth metals on the getter evaporates then redeposists as the flashing we often see. This flashing is able to bind stray gas for the life of the tube. The purpose of baking may be to reactivate the getter to a limited degree.* However, it should be noted that when I tried, I saw no new flashing. So perhaps another theory is that at >170 C, any resisdual reactant is revaporized and consequently performs more efficiently at bindng with any stray gas. If either is the case, the baking process really is a short-term quick fix. Just my musingsIt's not something I'd do on a regular basis. I probably wouldn't do it at all (nor rejuvenate tubes in the manner I described in my previous post).

*edit: in light of the completely overlooked fact that tubes run between 200-300C anyhow, this is unlikely. I wonder if it doesn't have something to do with how it's heated.

Keep in mind no vacuum is absolutely perfect, and it is common for tubes to slowly leak in air over the course of their life, particularly with use (in part due to the expansion and contraction due to heat shifts permitting miniscule amounts of air into the tube). Getter flashing physically changes over time and is clear evidence that no tube has a perfect vacuum.

 

[arr2036]

Ok , thanks for clearing that up Len

Just out of interest is the blue glow you see the stray gas being ionised in the same way that nixie tubes ionise the gas around the electrodes to make the characters glow ?

 

[drarthurwells]

Avoid turning tubed equipment on and off frequently.

I leave mine on for many hours at a time, and turn off only when I know I will not be using the euipment in 9 hours or more.

Sometimes it stays on for days.

This will prolong tube life better than anything.

 

[Len]

The blue glow is the ionizing of specific impurities, usually related to the glass envelope and harmless to the tubes. I had some KT88s that glowed extremely blue, though, so I tried the baking process with some success.

 

[kevin gilmore]

flashing the getter is a one time process typically done with an induction
heater which is a single loop of wire running at a frequency of
at least 10mhz. Usually 500 watts minimum of rf power, and for
bigger tubes more than that. The getter ring actually glows red
hot for the getter material to evaporate. In the range of 600 to 1000
degrees centigrade. Baking a tube at 325F is going to do absolutely
nothing as the inside temperature of most tubes is already more than
200C.

more info here
http://www.thevalvepage.com/valvetek/getter/getter.htm

 

[arr2036]

Certainly interesting reading ,

Quote:

In order to obtain a material of greater stability and safety in use than pure magnesium, the so-called Formier getter was developed, It consists of aluminum-magnesium alloy powder (55 percent Al, 45 percent Mg) which is applied suspended in a nitrocellulose binder and applied to tube parts as a paint. On account of the limited gettering powers of magnesium, Formier is used only when other types of getters with higher evaporation temperatures must avoided. Magnesium getters are difficult to degas, have little gas absorption up to temperatures of 175 C and absorb only oxygen. High vapor pressure precludes use in small tubes and at high operating temperatures.

Though that is only one type of Getter , as kevin said most would appear to be uneffected by heating .

 

[Len]

Quote:

Originally Posted by kevin gilmore flashing the getter is a one time process typically done with an induction heater which is a single loop of wire running at a frequency of at least 10mhz. Usually 500 watts minimum of rf power, and for bigger tubes more than that. The getter ring actually glows red hot for the getter material to evaporate. In the range of 600 to 1000 degrees centigrade. Baking a tube at 325F is going to do absolutely nothing as the inside temperature of most tubes is already more than 200C.

After baking tubes, I can get the blue glow to go away as well as improve gas readings on my various testers. I have no idea what accounts for this, but baking does something.

 

[jaymanaa]

Getter flash is something that does continue to do it's job as tubes get used, and it needs to be hot to function. The idea is that as tubes get used, the metal pieces will give off molecules that the getter will absorb thereby keeping the vacuum as hard as possible. As the tube gets used, the getter flash will recede, like my hairline did. Many of the old "critical duty" tubes even had markers on the getter lines with instructions to replace the tube when the getter faded past a certain point. A regular household oven can absolutely "re-activate" a getter, or get it hot enough for it to start absorbing molecules again. Some unscrupulous sellers even use induction heaters to glow a tube's innards to cherry red which does a couple things. It obviously makes the getter absorb gas, but also burns impurities off the plate and cathode which will give you a better emissions test score for long enough to sell the tube. This guy cracks me up;
I usually bake NOS tubes in a 350F oven overnight before I even power them up in a tester. BTW, many of your better tubes used a special wire on the pins where it went through the glass that was very close to expanding and contracting at the same rate as the glass. Most of your slow leaks are caused by tiny cracks around the tube pins caused by people wiggling the tubes excessively when pulling or inserting them. You usually have to have some pretty good magnification to see them. The best tube I can think of off hand to demonstrate getter wear, is the 7591. Their getter can get totally transparent after years of use and the tube will still work fine.