some of the newer preamp tubes do have a larger circumference than old preamp tubes. however, they *should* still fit. whereas the old tubes will have some space between the edge of the tube and the metal shield, the newer ones will sometimes actually touch the shield. just make sure the pins are lined up properly and push it straight in. it will feel snug but i've never encountered a tube that literally wouldn't fit. out of curiosity, what brand of tube is it?
oh, and this may be obvious, but when you're lining up the tube to put it in, look for the gap between the pins on the tube, and line that gap up with the gap in the holes in the socket. if you happen to bend a tube pin you can probably straighten it out with a pair of pliers and try again.
I wouldn't mess with trying to take out the metal shield surrounding the tube. it's going to be a hassle and it is most likely unnecessary to remove it. (if you do remove it, the outer screw-off shield will have nothing to screw on to)
also, if the tubes have been in there a while it's not a bad idea to pull them in and out of the socket a few times every once in a while. that helps clear up the contacts. sometimes the tube can be perfectly fine but it is not making good contact and that can introduce scratching and noise that can be mistaken for a bad tube. (you can also spray some contact cleaner in the socket when the tube is out for even more cleaning)
oh, and this may be obvious, but when you're lining up the tube to put it in, look for the gap between the pins on the tube, and line that gap up with the gap in the holes in the socket. if you happen to bend a tube pin you can probably straighten it out with a pair of pliers and try again.
I wouldn't mess with trying to take out the metal shield surrounding the tube. it's going to be a hassle and it is most likely unnecessary to remove it. (if you do remove it, the outer screw-off shield will have nothing to screw on to)
also, if the tubes have been in there a while it's not a bad idea to pull them in and out of the socket a few times every once in a while. that helps clear up the contacts. sometimes the tube can be perfectly fine but it is not making good contact and that can introduce scratching and noise that can be mistaken for a bad tube. (you can also spray some contact cleaner in the socket when the tube is out for even more cleaning)