shelf life of 'vintage' tubes

Discussion in 'Amps and Cabs' started by AlexF, Jan 24, 2006.

  1. AlexF

    AlexF Member

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    Its fairly obvious if you think about it that, eventually, the vacuum is going to be lost in any old tube one way or another. I accept it may be down to how and where they are kept, but is there any reliable info on what might be the ultimate typical shelf life for old tubes??
    Al
     
  2. Blue Strat

    Blue Strat Member

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    Why is it obvious that the vacuum will be lost?

    There's no data that I know of. The study would obviously take longer than 60 years since there are plenty of 60 year old tubes around which haven't lost vacuum yet ;)
     
  3. WailinGuy

    WailinGuy Member

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    For all intents and purposes, vacuum tubes don't age unless they are being used (or misused). The only danger I can think of to a tube sitting on a shelf is an earthquake.
     
  4. TubeAmpNut

    TubeAmpNut Member

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    You can bet vacuum tubes experience outgassing as well as bleed. That's not to say their shelf life isn't 300 years, however.

    BK
     
  5. mbratch

    mbratch Member

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    In theory, it's not a perfect vacuum and may have some bleed. But it is such a slight rate that I think the tube will last a very, very long time on the shelf. A successful NOS market for tubes has proven that out somewhat. From that you know it's at least a few decades. And tubes haven't been around long enough to prove much longer than that (yet).
     
  6. Mayflower

    Mayflower Supporting Member

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    Wrong.....Or strapped inside a Mesa Boogie cabinet as a spare getting the ever living hell rattled out of them by the volume and the cabinet resonating!!!
    That's what they did for alot of years and they became microphonic (spare 12ax7).
    I am guessing that is why they stopped doing it.
    Right Sixstringslut? He knows! (Mesa Boogie Ninja :hiP )
     
  7. davebc

    davebc Member

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    I remember reading an interview with C. Diaz where he was promoting the arguement that nos tubes had a shelf life.
    I respected him; but you can't prove it by me.
     
  8. Blue Strat

    Blue Strat Member

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    I think the quote was something like [tubes don't improve with age] which no rational person would disagree with. However, to interpret this to mean that they will degrade with age (based on Mr Diaz comments) is off base.
     
  9. mbratch

    mbratch Member

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    That's equivalent to the 'earth quake' scenario that Jim pointed out. ;)
    As long as the tubes aren't electronically OR mechanically stressed, they should be fine for a very, very long time.
     
  10. TheAmpNerd

    TheAmpNerd Member

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    Just to check this out. I took one of those "left overs" outta the
    packing case, then outta the box and tried it in my vibro champ.
    Curious to see of the vacuum was gone and if the tube would
    blow-up/melt-down/thermally-runaway before my eyes and
    what abusing it with my guitar.

    Well sorry to say it sound better then any of the new and
    nearly new production I've tried and seems to work just fine.

    I'm surprised that the box is holding up this well and the
    insert is intact...which reads:
    "TO TEST-LIFT TOP FLAP AND
    PUSH TUBE DOWNWARD"

    The outside of the box says the following:
    US ARMY - US NAVY
    TUBE JAN-CKR-6V6 VT-107
    CONTRACT NXSR-71297
    CONTRACTOR
    HARVEY RADIO LABS. INC.
    ACCEPTED FEVRUARY 1945 SC961A

    Perhaps I should dump them before all the vacuum has gone away.
    Funny it doesn't say Harvey on the tube either.

    I keep thinking, I hope I work this good when I'm 61 years old.
     
  11. drbob1

    drbob1 Silver Supporting Member

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    You can still get boocoo bucks for 300Bs manufactured in the 20s, that's over 80 years and going strong...
     
  12. RickC

    RickC Gold Supporting Member

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    This gives me an idea; I'll be auctioning off some MIA "hard vacuum" on ebay soon. Upgrade your inferior eastern block vacuum now! Tone for days. Special discount for TGP'ers, of course :AOK

    /rick
     
  13. bob-i

    bob-i Member

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    I'd like to understand more about the vacuum, not that it really matters as there's nothing I can do about it.

    If I remember right from Electronics school back in the last 60's, a vacuum tube isn't truly a vacuum but an inert gas is blown in forcing the oxygen out. Then a partial vacuum is created and the tube sealed.

    Now I'm seeing terms like "hard vacuum" and "soft vacuum". Can someone elaborate please?
    Thx
    Bob
     
  14. ross

    ross Silver Supporting Member

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    I am not a tube expert but I do know a little about vacuum.
    A vacuum means that there are fewer molecules than atmospheric pressure (~760 torr), and a perfect vacuum would mean no molecules at all. In a vacuum tube you have 2 concerns:
    1. Any gas in the tube need to be non-reactive (remove oxygen). A getter (usually titanium) is designed to remove mostly O2 along with other gases.
    2. The pressure needs to be in the right limits. If the pressure is too high, the electrical resistance between electrodes is high due to the large number of molecule that will impede the path of an electron. If the pressure is to low then the electron will have no conduction path and the energy (voltage) needed to move the electron across the gap would be very high.
    Tubes that are called “soft vacuum” are more toward the higher end of the pressure range for a working tube and a hard vacuum are toward the lower end of the range. I am sure that there are some tube experts that can add on to what I have here or correct me where I have made some assumptions.
     
  15. Roccaforte Amps

    Roccaforte Amps Member

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    and, some are still under water running strong that have never been replaced since the day they were put into service.

    That said, that doesn't mean every vacuum tube out there has the same success rate.
    I've bought thousands of NOS tubes, and I've had many bad ones due to leakage, and or from being tested in tube testers too many times.
    Its always a gamble when you buy this stuff unless you're buying from a reputable dealer who really cares about what he's selling.
    If the NOS tube is good, and is used properly, and the circuit its sitting in is correct, it can last many years.

    One last point I'll leave you with;
    Some NOS tubes were crap right from the factory, crap meaning they worked in some applications, and not for others. Never assume just because its "NOS", and it passes in a tube tester, its good for your amplifier. There are many that won't hold up.
    Some 6V6 tubes are an example of this. Doug
     
  16. stratopastor

    stratopastor Member

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    can I query this? In relative ignorance, I thought no conduction path is necessary, and them little electrons are flying through space because they were heated off the cathode and attracted by the positive plate... in other words, a perfect tube would include a perfect vacuum

    happy to be corrected!

    best wishes

    SP
     
  17. ross

    ross Silver Supporting Member

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    Stratopaster, I was trying to explain resistivity (1/conduction) in a vacuum. I know the physics involved but I know little of the engineering of a vacuum tube. A perfect vacuum may make for a perfect tube but voltages needed to move an electron may be so high that the end design becomes impractical. Did the original designers leave a small amount of gas to reduce the voltages? I do not know, what I do know is:
    Electrical conduction involves the ionization of air molecules across a gap. An electric field will cause molecules to ionize, and those ions will recombine with any electrons that may be nearby. When air pressure is high, ions recombine relatively quickly, because ejected electrons cannot go far away, since they are likely to collide with other nearby air molecules. When air pressure very low (vacuum), ejected electrons are likely to get farther away, and it will take longer for ions to find electrons to recombine with. So the recombination rate goes down, the equilibrium is pushed toward more ionization, and a smaller electric field can succeed in causing enough ionization to make a path for an arc.
    In a perfect vacuum, the distance between the molecules that become ionized, thus creating the "electron path" or conduction, gets so large that the electric field, the force needed to move an electron from point A to point B, becomes exponentially high, greater that a 100KV to force an electron across a 1 cm gap. A graph of the Electrical Field vs Air Pressure is called the Pashcen curve. A good example of this graph can be found at this web site:
    http://www.reynoldsindustries.com/product/2multipin/page17.asp
    I hope this helps.
     
  18. griley

    griley Member

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    I'm afraid you've made a mistake in understanding how tubes work. The Pashcen curve relates to arcing, which is not how tubes opperate. In a tube electrons are emitted from the cathode via thermionic emission (they get hot and jump off, for you non-physicists). They are then attracted to the plates due to the voltage applied. This current can flow perfectly happily in a vacuum and no atoms are required.

    It is desirable to have a high vacuum in a tube for a couple of reasons. Firstly any gas has the tendancy to react with the cathode and burn it out. Secondly arcing can occur, which leads to a large and uncontrolable current. I'm not sure which on of these is most critical but both are bad news.
    The need for a high vacuum is the reason that tubes have getters. These are used to produce the shiny getter flashing on the glass inside tubes. The flashing is made of a highly oxidizing material so that any left over gas reacts with this instead of the other components, keeping it out of the way of the tube opperation. You can see this happening if you have ever had the misfortune to crack a tube. The gas gets in and reacts with the flashing, turning it white.

    Stratopastor, you are right in thinking that a perfect vacuum is ideal, though not really possible in practice.

    Hope that explains thing a bit better, George
     
  19. ross

    ross Silver Supporting Member

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    George
    I gave a disclaimer in the beginning that I know nothing about vacuum tube design. Again, I was talking about conductivity in a vacuum. A perfect vacuum is a very good insulator. I realize that if you need an electron “cloud” created from the thermal emission of a filament for in a device like a tube to amplify a signal. I just did not know if the low pressure gas still in the tube was a necessary part of the tube design.

     
  20. stratopastor

    stratopastor Member

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    thanks guys... a day without learning is a day wasted...
     

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