Let's say I set the bias on an adjustable bias amp say to, 43. Then I replace the tubes, what makes me have to rebias it again, why does it not stay at 43? I'm really new to amp tech stuff as you can tell.
I'm new to biasing too, but I just learnt how to bias my new Marshall TSL. It came out of the shop with one side at 57mV and the other at 105.8mV when they should both be at 90mV.
Basically the reason you have to bias every time is because the tubes are not exactly the same even if they are the same type, eg EL34 or 5881's. There is some tolerance difference to compensate for. What amp do you have?