[Ernest]

I just put new tubes (12AU7A) in my old preamp.  How long do they have to run to be broken in?

Thanks,

Ernest

[mgalusha]

Ernest,

In my experience they start sounding tolerable in about 2 hours and after 20 hours or so seem to be settled down pretty well. I have heard some folks say 100 hours are needed. They may still change some after 20 hours but it's likely minor compared to the first few hours.

One thing that can affect this is how hard the tube is being run. At least I believe this to be true. I of course could be completely wrong. YMMV and all that.

Mike