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
