Vacuum Tube Break-in

I acquired a quad of Genlex KT88s for my VAC amp at one time to replace the factory tubes.  When I initially started using them I disliked the sound immediately and for the next 20 hours compared to the factory quad.  I ultimately swapped them out and put them away for a few months.  When I tried them again they were still not what I hoped for but suddenly over the next 10 hours they opened up and smoothed out. I thought I had given them enough time the first trial, but I eventually grew to really enjoy them.  Break in?  I don’t know, but many times there seemed to be improvement to be had in the first 20-40 hours of both power tubes and small signal tubes in my systems

The break-in period for tubes is real. At least 20hrs. but for true sonic improvement even more like 50 hrs or so depending on brand and tube.

If the tubes are new and not burnt-in before testing/matching them they will lose their closely matched condition after 20 hrs. The operation point and the plate voltage of the amp in which the tubes are used is an even more important factor for matching and how they will finally sound. Tubes should be matched for the plate voltage and quiescent current of the actual amp.Some companies do that (upscale audio as far as I know). Adjusting the bias after a while (if possible) is important.

there is always the conundrum with break-in/burn-in

is the gear or item burning in, or are we as listeners getting more accustomed to the sound over time, being exposed to it repeatedly? - we all hear our systems sounding different day to day, each time we turn it on... what we sense can vary, conditions can also change

age old question this is...

my sense of this, regarding tubes, is that for most well functioning tubes, they do take some time (in my view an hour to a day, or overnight) for their sound to stabilize, after that, our ears are also getting acclimatized, sometimes in a good way, sometimes not

@jjss49 So true in regard to listener acclimation period with any new component/variable being inserted into system. This especially true when I'm already contented with system sound quality.  My comfort zone is upended when something new placed in system, I'm in analytical mode which detracts from that comfort. Over time I can settle back into comfort zone, especially if change is positive. If change is negative, I find it hard to get back to comfort zone, in which case  I come to understand that particular change not good.

As for tubes, I find power tubes most sensitive to burn in, found at least 85-100 hours is time of great change.  Rectifier tubes sensitive as well, signal tube somewhat less. The fewer the tubes, the more I hear changes. SET, DHT especially sensitive.