In Search of the Perfect 300B Tube A Tale of Burn-In
The following tale is instructive. The first two 300Bs Western Electric sent me were said to have had 200 hours of factory burn-in. They were not shipped directly to me but first to another reviewer, in the US. When they finally arrived, I gave 'em a few days' rest, then 50 hours of additional burn-in before listening. Still, they were disappointing. I had heard new-production WEs before, so I knew how good they could be. Fortunately, I discovered a friend with a well burned-in pair of WEs. We arranged a comparison, and the shipped pair was definitely inferior.
But why? I checked with WE, who assured me that the tubes had been burned-in at the factory. (They sent me a new pair immediately, for me to burn in myself). Maltreatment by my predecessor? The tubes showed no sign of abuse. Production inconsistencies? That seemed improbable with WE! I put them aside for a while, and, when the burn-in scheme allowed, gave them another 200 hours.
In the final listening session we compared them again against the second (and, by then, well run-in) pair, and against my friend's pair: the difference had vanished.
We'll never know the truth of it, but I wonder what results my US colleague got, if he even listened—but with a 300B, I would guess he did. I don't know his name or the magazine he writes for, otherwise I would tell you.—Peter van Willenswaard