tube question

Try the audioasylum.com, many tube gurus hang out over there.

There are several pages on the web that give you the prescice data for the tubes tolerances and what you can sub . As mentioned I always do a search on Audio Asylum especially their tube group, first to see if the "pin out" and voltage is very close to the tube I am going to try.

It's not the designation that's so important as long as you use a 5Y3. It's the brand. Different brands of rectifiers (which the 5Y3 is) sound different from each other and affect the sound just as much as different brands in the audio stages.

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".

You might want to buy one of these:

http://www.amusicdirect.com/products/detail.asp?sku=BM12

Cheers