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Author Topic: New tube mic burn in time  (Read 3485 times)

musiclab

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New tube mic burn in time
« on: August 08, 2011, 08:30:59 pm »

I read something on the net that quoted Klaus Heyne telling someone that before they sent their tube mic in to be modded
they should have it on and plugged in for a number of days, [I think 7]. If that is correct should they be on for 24 hours for the 7 days. Pardon my ignorance but I never bought a new tube mic before, usually I get them used. Last week I bought 2
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klaus

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Re: New tube mic burn in time
« Reply #1 on: August 08, 2011, 09:54:28 pm »

What you read may have been information quoted out of context. In my original comments I had referred to replacement tube installations in existing, not new, mics (in the KHE-model, I used the same method.)

I assume that a tube mic manufacturer would use proper methods to weed out bad tubes- those that would fail immediately or prematurely- before they would be installed in a new mic going out for sale.

Having found that a new tube (a high-quality NOS tube, to be sure) will not change much after ca. five to seven days continuous run, I am able to reasonably eliminate the highest percentage of eventual failures, once the mic is in the field.

If you were to look at a graph with the x-axis as time, and the y-axis failure- either steady noise, discharge noise or vacuum leaks, you would notice that after about a week of burn-in, the curve has fattened out to a straight line. Any failures after the first week of continuos operation will be so random and unpredictable that it serves no practical purpose to continue a test.

From experience,  tube failures happen: 90% within the first five minutes, 5% within a day, the rest spread out over the next five to seven days.
Different tubes fail at different times; for example, 90% of an AC701 will fail within one day, rather five minutes. Other tubes have a different failure profile.


Your second question: can you improve the sound of a tube by "burning it in"? Maybe marginally. The reason why this is hard to tell:  it is often hard to distinguish which component is responsible for the improved sound after a few days' of a mic's operation. Was it the coupling capacitor forming? The supply voltages stabilizing? The tube?

Other changes can happen within the first few days of operating a new tube: filament deposits need to burn off,  with the effect that, what initially sounded like a noisy 6072, say, will turn out beautifully after a day; or a crackling AC 701 may lose its discharges with time as well, then operate quietly ever after.

Please read more about this subject here: http://repforums.prosoundweb.com/index.php/topic,1141.0.html
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com
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