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How Long to Burn-In Western Electric 300Bs?

I have a new pair of reissue Western Electric 300Bs that I am running in my Morrow Audio SET amps. I read some comments in a Stereophile column published a number of years ago in which the reviewer discussed a very protracted burn-in time for the tubes. The reviewer had experienced a dramatic change in the sound of his WE 300Bs after a burn-in period of several hundred hours. He later ran into the principal at Western Electric (Charles Whitener) at a trade show and inquired about the manufacturer's recommendations with respect to burn-in of the tubes. In a follow-up column, the reviewer relayed Western Electric's suggestion that the tubes require a minimum of 500 hours of burn-in.

The Stereophile follow-up column goes on to say that the Western Electric 300Bs require a much longer burn-in period than most tubes because WE chooses not to use calcium oxide in their filament coating. WE apparently believes that calcium oxide shortens the filament life. The downside of this approach is that it apparently takes a LONG time for the filament to burn in without the calcium oxide acting to accelerate the activation of the barium in the filament coating.

For those of you running Western Electric 300Bs in single-ended amps, I would be interested in your experience with respect to burn-in. Were there truly significant changes in the sound of the WE tubes hundreds of hours into the burn-in process?

Frankly, I felt that the WE tubes sounded excellent right out of the box, and I can't imagine there will be significant improvements. Nonetheless, I'm curious about others' experience.
cincy_bob
raquel
841 posts
 
My guess is that WeCo's explanations as to break-in for the tube are true. The break-in process is not exactly a crowd pleaser, and perhaps for this reason, my experience is that manufacturers tend to understate break-in time if anything.
cincy_bob OP
270 posts
 
Well, at this point, I have about 120 hours on the new pair of Western Electric 300Bs. The sound of the tubes is definitely changing with burn-in. Fortunately, the bright characteristic I mentioned above began to noticeably dissipate at around the 80 to 100 hour mark. In listening to a string quartet this morning, I found that the peakiness in the middle to upper register of the violin is much more tolerable but not altogether gone at this stage of the game.

It will be interesting to see whether the tubes continue to change with additional burn-in. More news to come...
cincy_bob OP
270 posts
 
I'm checking back into this thread now with about 340 hours on my pair of Western Electric 300Bs. Regarding the brightness or resonant glare that I heard at certain frequencies in the middle to upper register of a violin, that problem has now completely resolved itself with burn-in. String quartets now make for pleasant listening. (No more fingernails on a chalkboard during peak passages featuring the violins.) My pair of 300Bs seemed to hit a plateau at around the 250 to 300-hour mark.

So, based on my experience in running the Western Electric 300Bs as output tubes in the Morrow SET amps, it appears the tubes really do require an extended burn-in time on the order of 250 to 300 hours to sound their best.

I will check back in here if I notice any further evolution in the sound of the tubes.
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