tube burn mcintosh pre-ampIs

     Here are just a few sentences (verbatim), from a letter that Brent Jesse includes, when you receive tubes purchased from him:

     "New tubes need a break-in period before they can exhibit their true sonic character when used in audio circuits. Preamp tubes especially benefit from a good break-in. This period may vary widely, but NOS vintage tubes usually need at least 48 hours of use, sometimes up to 100 hours. New current production tubes need 24-48 hours typically. Good break-in can be accomplished by either leaving your tube unit turned on for 2 days in a no signal condition, or you can just enjoy the tubes for several hours each evening and they will be broken in after several weeks."

     interesting: https://www.youtube.com/watch?reload=9&v=jwmDf5bSRMQ

I just like to play FM in the background when I burn a new piece in. For me it takes the placebo effect out of the equation because I’m not being critical about what I hear and it lets me acclimate to the possible change in sound.   

IME any tubes MUST burn in for at least 24 hours before you can be sure that what you're hearing is a fair representation of what the tubes' sound will be. For small tubes much change beyond 24 hours will be small (unless you're super anal) power tubes can take a few more days.  FWIW.