Yesterday I replaced the Sovtek 12Ax7 with a GT12AX7-M in my Hartke 5000. When I powered up I heard a loud farting sound every 5 seconds or so. I called GT and they said that it was probably a bad tube. Today, I powered up and the sound is gone. Anyone have a similar experience? Does this tube need to break in? I am worried that it might go bad again. The thing sounds bitchen! Really warms up the tone. Don't even need my Sansamp as much now.