I have a Mesa Lonestar Classic 2x12 using Mesa 6L6 tubes. I usually run it on 50 Watts for both channels, but regardless of settings (100/50/10 watts) that one power tube takes a little longer to start glowing. I know its not the rectifier tube either. I don't hear any difference in sound either. Big show this weekend and can't afford to get new power tubes and have them biased right now. But long story short....3 of the 4 power tubes glow immediately. One of them takes longer to start glowing. Not sure how long, but if the amp has been on for a little while, I'll look and they're all lit up.