I just scored a nice deal on a 6-month old PF-50T, and I've been playing with the bias setting on the back panel. The manual says you can set it for a fat tone and short tube life, or you can set it for a thin-sounding tone and long tube life... all from going by the red/green LED's. I can hear the tonal difference depending on where I set the bias pot. There are a few things I don't get, though. 1. Is the bias pot supposed to rotate 'round and 'round instead of a definite stop and start point, like on a typical volume or tone pot? 2. How long could I expect my power tubes to last if I adjust bias for "better tone, just before the red LED lights up" (according to the manual, page 12), assuming two 4-hour gigs a week? 3. What the heck is bias anyway? I know it's a setting peculiar to tube amps, but I always thought it was an adjustment done by a service technician. 4. Does the bias setting drift over time, or will it stay where I set it? You can tell I'm new to this stuff, I just need some 'splaining. Thanks in advance.