A little history. I bought the Crack in 2015, built it successfully then started the speedball. About 1/2 way through I moved homes and just last week finished it. It was a bit messy trying to clean it up, figure out where to pick up and complete, but nonetheless, everything seemed to work out well. That is, until I started noticing a crack/pop in the left channel. Sometimes silence but only in the first 15 minutes of listening. Once I get it fired up and maybe give it a little tap on the top plate, everything works perfectly. I did notice that it seems to occur again when I wiggle the 12AU7 tube, so I replaced it with a Mullard - same thing but only within the first 15 minutes. I'm listening now and it sounds amazing with Sennhesier HD650 and 58X (btw BUY THESE - HOLY MOLY they are fun with Crack even at 150ohms), but I'm a bit concerned that something is not right. I'm wondering if I have a bad solder joint somewhere. I have no idea where to start diagnostically after searching forums. Is this a tube warm up thing? A bad solder joint? Any ideas or starting points for a newbie??