If it's a manual bias you need to know how to bias them first and formost. Only because I have never owned a tube that didn't need biasing after replacement. No matter what tube I 'roll' including input and driver tubes I always start at min bias to let them all warm up first (about 1/2 hr). Then I bias them low and check the bias every 1/2 hr or so. Then I check again on a daily basis for the next few days. I have seen tubes take several days to settle down and stay properly biased.
That being said I will admit that 'factory' matched tubes can be very poorly matched. Something I learned from Jim McShane when I bought some GL KT88's and SED EL34's. I brought my 'factory' matched EL34's to Jim and he showed me how far away they were from a 'good' match.
Good Luck
That being said I will admit that 'factory' matched tubes can be very poorly matched. Something I learned from Jim McShane when I bought some GL KT88's and SED EL34's. I brought my 'factory' matched EL34's to Jim and he showed me how far away they were from a 'good' match.
Good Luck