Which they are, but fixed bias means they are set to a particular bias point. A fixed bias amp can have an adjustable bias if it has a bias pot. Without a bias pot a fixed bias amp's bias will fall where it may unless a bias pot is there or added to adjust the bias. Fixed bias doesn't mean "self-adjusting". Cathode biased amps are much more "self-adjusting", but even those should be checked as I have seen a few that ran too hot. Most Mesa's and Peavey's are fix bias, non-adjustable, but not all.
When purchasing tubes for these fix biased, non-adjustable amps it is recommended to tell the tube reseller what amp the tubes are going in so they can make sure you get cooler tubes. I have had really hot tubes come from tube resellers and these tubes would have run really hot in a fixed biased, non-adjustable amp. Fortunately, most of these type of amps (Peavey/Mesa) have been setup with their bias pretty cold to compensate for hotter tubes, but it doesn't always work out this way. My Valveking head runs pretty hot as an example. Hotter than I think Peavey intended.
