Looking for some picture tube help

[leadlike]

Hello folks, I'm very new to the forum but figured that since I've been reading it for so long I'd better join up. I have an RCA 21'' from 1955, it is the only really vintage tv that I use and we just love it. Sadly, that old 21amp4a is wearing out, and I need to find a suitable replacement. I don't mind doing modifictations and the like to get a new tube in, but I would really like to know what my options are with this set. I don't have the chasis number with me at the moment, but would be happy to supply it if it would be helpful in finding a substitute. Thanks in advance for all your help.