Do tubes need break in / burn in?

I have found about 40-50 hours typical are required for break in of tubes. Some brands and types more, some less. It could be that the new tubes you put in are just not a good match for the preamp. I've had a couple of different pieces of gear that took like 8-10 tries to find the right tube. It's a crap shoot for sure on some gear.

This has been discussed here before. Type in nos tubes in the "search archives" section of this forum and you will see a posted question from 2008 about this same subject.

HI, had same preamp years ago,VTL5.5 and the only tubes that work best was siemens 12ax7 don't even try telefunken they sound terrible in that preamp, another suggestion is that,
VTL 5.5 is not a good match with SS, amps reason why sold my

regards: ,

12AX7 has a gain factor of 100, while a 5751 has a gain factor of 70, which may be the reason that your preamp sounds so poorly. Try using some NOS 12AX7's instead of the 5751's.

In general burn in will take the edges off. However the sound after about 10 to 20 hours is going to represent the end product to a great extent.
Your pre may not have a circuit they will work in as mentioned above. A CD player I bought early in the hobby just made a GAWD awful shriek of painful protest when I subbed them.
I recognize that many will argue that a tube fleshes out with time and it's true, but that takes place gradually over a long period. In the interim, however, the tube will show what it is going to be.
To me immediate burn in has almost always been a smothing up with some getting significantly fuller tones. However Flat earth to halographic soundstage is not what I have experienced.
It really sounds like a really bad pair of 5751s.
When they do work out, they function beautifully as a stage expanders with halographic imagery. I started with Jans and they sent me sliding down the slippery collecting slope.