Just wanted some input from all you vintage tube amp lovers out there. When and how often do you replace your power tubes? (I know this depends on usage) How can you tell if a power tube is in need of replacing? I have a late 70's V4B with the orginal tubes in it. I was thinking of taking it to a tech to give it a check up, and make sure everything was working properly. So, I was thinking of having the power tubes replaced and rebiased at this time. Should I keep the orginal tubes in there as long as possible to preserve the "vintage tone" or would replacing them give me a more optimal sounding amp? Also, any prefrence on the Sovetek or JJ Tesla 7027s?