will i have to re bias my amp if i change the tube in my amp to the exact medals that where already in like el34-for-el34 and ecc83-for-ecc83
sometimes you will have to, its good practice to at least check your idle current to see if everything is ok

and you dont have to bias preamp tubes, just the el34's

and if you dont know how to check/change your bias, look your amp up, there's a few ways to do it
if you keep the same tubes everything should stay the same and I dont think you should have to rebias
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it'll be alright, but if you notice anything wrong with your amp/tubes (excessive glowing, it just isn't sounding right, etc) check your tubes and then your bias.