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1-10 of 17 messages
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Vacuum tube burn-in
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by W5RKL on May 16, 2008
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I have heard from various hams that state if the
tubes in an amplifier are replaced with "new" tubes,
the amplifier should be run for 8 to 12 hours
"before" it is used.
I have a few questions about this and the questions
are:
1. Why is this 8 to 12 hour burn-in so important?
2. What is accomplished by doing an 8 to 12 hour
burn-in?
3. What actually takes place during the 8 to 12 hour
burn-in process?
4. What ill affects, if any, will occur if a "new"
tube is not burned in for 8 to 12 before using it?
I honestly don't see the purpose behind doing it.
I have never done this and by not doing it I've
never had an early tube failure or a change in
output power, grid current, etc by not doing it.
If part of the answer is:
"Removes gas from the tube"
Please explain how this is accomplished with only
the filaments lit.
If part of the answer is:
"It completes the vacuum process"
Please explain how the vacuum process is completed
with only the filaments lit.
If part of the answer is:
"Allows a more even distribution of emitted electrons
from the cathode/filament"
Please explain what changes in the cathode/filament
occur to produces a more even distribution of
emitted electrons by running the filament for 8 to
12 hour.
If you have any additional information pertaining to
this topic, please include it.
From all the answers I've received so far about this
topic, they all boil down to this single answer:
"I heard it from a number of hams that it needs to
be done so I do it."
Yet when asked why it's important to do it and what
benefits, if any, are achieve by doing it, no one
knew any, they just do it because they heard others
do it.
Thank you for your answers. Let us all learn something
from this topic.
73s
Mike
W5RKL
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RE: Vacuum tube burn-in
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by W8JI on May 18, 2008
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Any benefit of filament burn in depends on the tube type and age. High power tubes generally have a gettering system that keeps the tube free of gas. This is because seals sometimes leak and materials used inside the tube can slowly release trapped gas.
Nearly all gettering materials used in big tubes requires significant heat to activate.
External anode tubes like the 4CX250, 3CX1200 or 8877 have the gettering materials mounted on the filament or cathode structure. They are degassed by heat from normal filament temperatures.
Internal anode tubes like 3-500, 4-400 or 3-1000 types generally have the gettering materials on the ANODE. It is the gray powdery looking coating you see on the anode.
Obviously you cannot getter or degass a 3-500Z by running the fialment. You have to operate the tube until the anode shows some color in order for the getter to clean up the tube. This is why storing tubes like the 3-500Z or 4-1000A for extended times is a bad idea and can ruin a tube. It is also bad to NEVER show anode color while operating.
On the other hand a tube like a 8877 or 3CX1200 can be gettered just by running the filament, so it might be wise to run a tube that has been out of service for some period of time, say a half hour or longer, to allow the gettering to work. You should NOT have HV on the tube during that time.
So you see, the answer is complex and varies with the particular tube. With some tubes it is a total waste of time. With other tubes that have been laying around unused it can serve a purpose.
Try reading:
http://www.w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm
73 Tom
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RE: Vacuum tube burn-in
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by W5RKL on May 18, 2008
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Hi Tom,
I agree with your response.
I read your websites article on high power tubes and
have followed the recommendations for quite some time
now.
The amplifier's in question used 811A or 3-500Z tubes.
The Zirconium getter anode coating on these tubes will
never reach the 1000 degrees cent temperature with
filaments only lit.
Anyway, thank you Tom for your response.
73
Mike
W5RKL
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RE: Vacuum tube burn-in
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by W6CCD on May 18, 2008
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I looked through some of my old manuals this morning. The only thing I can find regarding burn-in is in the RCA Transmitting Tubes Technical Manual TT-5. In the case of a mercury-vapor rectifier, spattering of the mercury from handling can result in internal shorts. If spattering has occured, the filament should be run for 30 minutes to assure complete vaporization of the mercury content. Doesn't answer your question re amplifier tubes.
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RE: Vacuum tube burn-in
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by K5END on May 23, 2008
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THESE DAYS...
it's a good idea to heat the tubes up and allow them a chance to fail before you put high voltage on them.
Of course that doesn't test the tube operation at high voltage. Without a test bench, a nice Hickock tester or other test equipment, you can use your other senses to identify major problems.
Once the tube has run current on the filaments for a while, with no glowing plates, no popping or flashing, etc, I visually observe the tube when high voltage is applied:
First, if you see a very pretty cobalt-blue or purplish glow, it is because air has leaked into the tube. That beautiful color is associated with the energy states of the electrons dictated by and specific to the elements in the gas, most of which is nitrogen. You are witnessing quantum mechanics, sort of.
Second, if the tube innards change color soon after the high voltage is applied, especially orange or red, turn off the whole rig immediately and toss the tube. It is probably shorted.
If you observe sputtered material or loose parts within the tube, toss it. If the glass interior is or turns white, toss it.
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RE: Vacuum tube burn-in
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by W8JI on May 24, 2008
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The question was about amplifier tubes.
Very few amplifier tubes can be tested in a tube tester.
A red anode is NOT a sign of a bad tube. It actually is normal for some tubes to run red hot on the anode, as a matter of fact many tubes will FAIL prematurely if they are not run with red anodes. These are the tubes like the 3-500Z and 4-400 and other tubes with directly heated filaments and internal anodes. The anodes have a Zirconium coating that getters the tube, and the anode must be heated to the point where it has some red color to activate the zirconium getter.
Also a faint blue glow can come from harmless fluorescence in the glass, not just from gas.
Running the filament doesn't do anything in tubes with an anode gettering system. It mostly helps only in high power external anode tubes. External anode tubes typically have the gettering materials located on the filament or cathode structure so filament heat activates the getter.
73 Tom
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RE: Vacuum tube burn-in
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by K5END on May 26, 2008
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You make a good point about power amplifier tubes as compared to the others.
However, I was referring to how the quality control of new tubes from certain vendors/manufacturers on the market has gone, uh, down the tubes, so to speak, or NIB tubes that may be decades old. Pardon the confusion.
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RE: Vacuum tube burn-in
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by RADIOFREEISRAEL on May 29, 2008
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Fascinating subject and as I read the answers it makes me go back to Rauland and Dumon years...
Thermoionics is a subject aprt of our College education but once one gets down to designing, manufacturing and using tubes it turns into a fantastic setting.
We designed and made electron guns and CRT's but participated in the design and application of medium power transmitting tubes. From two to 10 kilowatt induction heaters for example.
The kathode and filament processing as well as the outgassing of the metal elements in an electron gun and the processing of the power tube have much in common and after all, why not?
The same Laws apply to both of them.
Outgassing of the K/F region is accomplished by carefully energizing the filament as difussion and mechanical vacuum pumps reach out for the remnants of process gasses. The K activation process in the indirectly heated cathode systems is entirely thermal.
Both the chemical processes and outgassing of intermolecular gasses are so effected.
That while or shortly after the region is "bombarded", heated via induction heaters working in the region of 500KC.
Both tubes and CRT's go through similar processes.
Gettering is entirely a delayed process and it could be part of the filament or "flashed" or deposited soemwhere within the envelopp or parts.
ENTER the nemesis.
There are no perfectly sealed containers. None.
Leaks into a vacuum chamber occur. Point. It is all a matter of rate of leak not of abscense.
Consequently no tube should be stored innactive for long periods and the snapped into activity w/o a healthy warm up period.
At Rauland I developped a kathode image process that projected the K image into a screen to obaserve ion and electron transit. Ion currents hitting a cathode as the anode/s are snapped on are deadly. (AKA ion burns) on that side not only on the screen side.
Mottled, pitted, even perforated cathodes are the result.
We can go one for hours on this and I love it but at least my experience is that of those that spoke before me quite well.
Tubes "like" to work hot... but dislike being turned on to full power after long siestas... LOL
BURN IN your tubes even more so now that the tubes you may buy have been "carefully" stored for decades before going to work.
Great topic.
Sam
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RE: Vacuum tube burn-in
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by W8JI on May 30, 2008
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Sam,
I didn't want to confuse people by describing what is done in the manufacturing process when the tube is pumped down.
The question was about what to do with higher power tubes typically used for linears.
The answer is:
If the tube has an external anode like the 3CX1200, 8877, 3cx1500A7, 8873, 4CX250B, 4X150A, GU7B, and so on it almost certainly has the gettering system located in the area of the heater or filament so cathode or filament temperature will heat and activate the getter. Running the filament for a period of time without HV will often degass and help prepare the tube for operation.
If the tube has an internal anode that is coated with a powdery gray coating (zirconium) like the 572B, 3-500Z, 4-400A, 3-1000Z, 811A, and so on you can run the filament until the cows come home and essentially nothing will change. This is because the zirconium that getters the tube is coated on the anode, and has to be heated to the point where the anode is red or nearly red to work. The filament can't do that, so running the filament to burn the tube in is generally just a waste of time.
As a matter of fact 3-500Z's and other tubes can go bad or have short life failures if the anode is never heated sufficently to activate the getter.
Power grid triode are indeed manufactured the way you describe. They are generally on a rotating table with high anode current and high filament voltage applied to heat the tube internals while a vacuum pump pulls the tube down. Eimac even rotates the tubes through an oven where they are heated externally as well as internally in the pumping process.
Running the filament won't do much once the tube is sealed unless it is an external anode tube that has the gettering agent located in the area of the filament. It can, on occasion, actually hurt the tube by outgassing the cathode area if it is a tube that does not use the cathode heat for gettering.
73 Tom
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RE: Vacuum tube burn-in
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by RADIOFREEISRAEL on May 30, 2008
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Hi Tom,
I fully agree with you as I mentioned before.
Power tubes are very much sensitive to how we operate them once tipped off after exhaust.
Production lines for some of them include UHV stabilitzation, getter flashing or activation and kathode/filament ageing.
The post exhaust processing does not end there though.
When we introduce them to working conditions the processing in a way continues as you properly indicate.
The plate and certain grids certainly benefit from being worked at adequate temperatures and again as you mention, preventing them for reaching soft red temperature is not really good for those tubes.
Being as it is that many younger engineers or technicians are joining the great world we lived in for many years, I would love to be able to provide for the newcomers some resources that will help them and why not, all of us, gaining knowledge from their and our experience.
Some of the best people from Westinghouss, RCA, Sylvania, Zenith, Raytheon, EIMAC, and many others accumulated huge knowlege on this.
I do not remember who mentioned that "testing" power tubes on even the best Hickok tester ever, is not really significant. True to certain degree.
BTW. Does anyone know where one can find one of those OLD gas ratio test units made by Griffith in the 40's and 50's?
This is a great subject.
Sam
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