I've heard them change right before my very ears...when recently testing out a potential purchase, the seller had mentioned that he had purchased new JJ Tesla kt88's with approx 1 hour's time on them if that. As the amp was warming up with music playing both of us were sitting and remarking at how "hollow" and "cold" the sound was as the same track was repeated itself in the background.
Over the course of about 45 mins as we chatted about birthcities and other irrelevant issues I realized that I needed to make a decision re: the purchase of this amp.
We replayed some of the previous tracks and both heard an amazing amount of difference, the sonic characteristics had changed drastically over the 45mins of additional playtime. So much so that it may have worked against my negotiation tactics.
Now, this could be marked off as amp warm-up vs. break-in but I'm sure the amp was already warm 20 minutes after I had arrived.
With what I heard and continue to hear, that small amount of time made a HUGE difference with sound.
Total play time on the tubes when I heard the difference was approx 2 - 2 1/2 hours...hopefully it gets even better.
I'm still playing with triode vs. pentode but both sound remarkably better than when I first listened to my new baby.
HTH, fwiw I've had no previous experience with tube break-in.
Happy Holidays!
