Possible subs according to Rider.
E = excellent match
G = good match
5ar4 E
5ax4 G
5cg4 G
5t4 G
5U4 G
5v4 G
5w4 G
5y3WGT E
5z4 G
5931 G
6087 G
6106 G
6853GT E
TE22 E
TE45 E
U50 E
Many tubes will be followed by G, GA, GB, GT, WGT and so on...Usually this is an indicator of the tube's ability to handle a little more voltage (only if your amp can supply it).
Also, this is a rectifier tube. Unless you need one that can handle extra voltage or abuse, don't change them. Your money will be much better spent on gain or power tubes.
For a great reference, go to www.sonicfrontiers.com and read several articles including "a taste of tubes".