Do i need to rebias my Peavey 5150 when I change Tubes or do I have to do I have to rebias it at all? I read that it has something called a fixed bias. If that's the case can I just swap what ever brand of tubes I want into the amp as long as they're the same type (eg 12AX7s, etc)
The 5150 can't be biased unless it's modded. So, no. You don't have to bias preamp tubes anyway, but in the 5150, the power tubes are just plug and play.
Pretty much what Matrix said. If your trying to replace the 6L6's with 6L6's, you don't need to do anything but plug then play
What they said
I really think that is a horrible method for the amp and would really recommend everyone install a trim pot in the bias circuit so that they can at least have some actual control over setting the bias instead of just hoping the power tubes they install draw enough current.
^ Same here, actually. I have no experience with the amp, but have heard from multiple people that the 5150 is biased very cold, and if you set it higher (within reason, maybe 3-5mv), it really brings new life to the amp.