My Mark V seems to be making a crackling noise that responds with pick attack when I play it for the first 20 minutes or so. It starts to have a whirling tremolo effect and the volume just fluctuates but it eventually goes back to normal. I've owned the amp for about 8months now and whenever I play it I usually keep the volume around 10:00-11:30. Are my power tubes failing? I checked the tubes when I hear the crackle but everything looks normal. My jam room is in my cold garage but I always let it warmup in standby for around 15min before playing. What do you guys think? Might be the time for some new SED Winged =C= 6L6s!