I just swapped for a Blankenship Fatboy w/ Sour Cream. It's tweed amp w/ 2x6L6 power. In addition, it has two 12Ax7s, two 12AT7s, and one 5AR4 (GZ34) rectifier. Turn on, plug in, and the amp sounds great. After a few minutes, though, there's a distinct change on tone. All of the warmth is gone, a fair amount of volume is gone, and what's left is "ice picky bright" sound that also sounds like its coming through cupped hands or a megaphone. A couple of minutes later, the warm, full-bodied tone returns. Would appreciate if someone who's had an amp w/ these symptoms could chime in w/ an "educated diagnosis."