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.