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    'Breaking-in' power tubes

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    deepee99

    Posts : 2030
    Join date : 2012-05-23
    Location : Wallace, Idaho

    'Breaking-in' power tubes

    Post by deepee99 on Fri Nov 02, 2018 2:10 pm

    Is there any rationale for breaking in new output tubes, or is that just some snake-oil I read on another site? And if new output tubes *do* need a break-in, is there an accepted protocol?
    Thanks,
    Dave
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    Peter W.

    Posts : 965
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: 'Breaking-in' power tubes

    Post by Peter W. on Fri Nov 02, 2018 2:36 pm

    deepee99 wrote:Is there any rationale for breaking in new output tubes, or is that just some snake-oil I read on another site? And if new output tubes *do* need a break-in, is there an accepted protocol?
    Thanks,
    Dave

    Back in the days of pre-blight tubes, there was some rationale in the concept of break-in. Tubes were made, getters were flashed and then the tube was tested for quality prior to release. The "Break-in" rationale was based on the innards of the tube stabilizing and any residual 'stuff' getting burnt away. And, the more robust the tube, the greater the likelihood of some period of break-in. Also, those tubes (let's focus on power-pentodes) were expected to last well over 10,000 hours at the short end. So, a couple of hours of break-in was the equivalent of 0.002% of the expected service life.

    Today, even optimists put post-blight power-pentode average life at about 2,000 hours, 20% of pre-blight. That would suggest a break-in period of about 24 minutes.

    Note that there are some 'real things' out there:

    a) Lazy Cathode: If you are going to break in a tube, do it by actually running it. Leaving a tube with a hot filament and no signal can (not will) contaminate the cathode, requiring burn-off when the signal is provided. Small-signal tubes, especially oscillators, are particularly prone to lazy-cathode issues. Pentodes not so much. Lazy cathode takes HOURS, not minutes, by the way. Such as an FM oscillator tube being kept hot in an AM/FM radio where the owner never listened to FM.
    b) As long as it is being operated within design parameters, a tube does not care how hard it is run. These are not automobile engines that need variation in speed and load to break in properly.

    Now, here is the way of it - WERE power tubes to require any sort of substantial break-in, then matching would have to be done after that break-in. Which would add significantly to the cost of said tubes as well as shorten their actual in-service life.

    And, on matching: Tubes drift. Post-blight tubes drift far more than pre-blight tubes, but all of them drift. So, it is not a bad idea to hold on to odd tubes to re-match in the future - if you are so-equipped. Tube should be matched within a channel. but there is no electrical need for them to be matched across channels. Yes, their may be some fractional difference in output between channels, but, unless you are a sweet-spot fanatic or listen only to headphones - not so's you'd notice.
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    peterh

    Posts : 1054
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: 'Breaking-in' power tubes

    Post by peterh on Fri Nov 02, 2018 4:51 pm

    deepee99 wrote:Is there any rationale for breaking in new output tubes, or is that just some snake-oil I read on another site? And if new output tubes *do* need a break-in, is there an accepted protocol?
    Thanks,
    Dave
    No.
    Emission might change slightly during the first hour or so, but then no surprises are expected.

    Myself use a quad of JJ 6550 that has 4091h in use and still holds bias.



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