> "cardas quad eucitic solder"
Eutectic is easier to use, for some people. http://en.wikipedia.org/wiki/Eutectic_point
Water goes from solid ice to liquid water.
50:50 lead-tin solder goes through a "paste" phase, not liquid not solid. This is very useful for "shaping" a joint, as in some plumbing and roofing soldering.
Anybody here know Apple Jack? Squeeze apple juice. Let it ferment to Hard Cider. This is ~~3% alcohol and the rest is water (and color/flavor). Put it out on the porch in January. If you catch it right, it turns to "slush". The water is frozen hard but the alcohol is still liquid. Pour the alcohol into a liquor jug, leaving the water behind. This gives a higher-proof (though not high-proof compared to distilling) spirit beverage. 50:50 solder goes through such a "slush" phase.
Also, the more lead and less tin, the lower the cost. Lead is cheap, tin is very expensive. If you are packing a 4-inch sewer pipe joint, you need a pound of filler, low-cost is important.
In hand-soldering, cost of solder is NOT important. (That does not mean you should buy $5 solder for $25 just because it has a fancy brand name.)
Traditional electronic solder is 60% tin 40% lead. It has a low melting point. It has a small "pasty phase". This can be used, by experienced workers, to smooth the joint. However, inexperienced workers tend to move the wire while the joint is half-hard, which gives a joint which may look good (if you don't know what to look for) but is really all broken-up under the surface.
63:37 solder has a slightly lower melting point, but also it goes from liquid to solid with almost no pasty phase.
Wikipedia claims that 63:37 is the alloy of choice for electronics assembly, and that 60:40 is nearly the same. I recall when 60:40 was the only common alloy, and do not agree that it is "almost identical" to 63:37. Close, but not the same. It is like making biscuits: as you get close to an "ideal proportion", small changes matter. 60:40 is off-eutectic just enough to matter.
I agree with reducing the amount of Lead in our land-fills. I believe the amount of Lead used in hobby electronics is insignificant. I do not like no-Lead solder. Anyway I have a lifetime supply of the leaded stuff. I think most hobbyists may as well use the Lead solder. Buy 4 ounces at a time. You do not want to, like me, die with a couple pounds of unused lead for your heirs to clean up. When you find yourself on your second or third small spool of leaded solder, you might want to try no-lead... you are no longer an occasional solderer.
If you have children around, do NOT NOT NOT let them play with solder. If your children are as sneaky as I was as a child, you should probably remove all lead solder from the house and stick with no-lead. Even that is not 101% safe (all metals have some toxicity), and should be kept out of reach or warned-against as much as possible.
Apparently there are the older high-temp no-lead solders (which must be used for plumbing; I can't get lead plumbing solder); and some lower temperature no-lead solders for electronics which have a high Silver content. Since Silver is even more expensive than Tin (not a lot more), this must be a burden on high-volume hand-soldering. The amount of solder an amp-hobbyist uses, the cost of Silver is a non-issue.
Solder does not smell. Flux stinks. The classic "rosin" is highly purified pine-tree sap. It is a very complicated mix of organic compounds, nobody really knows what-all is in there. It works. It has a familiar smell. It is generally accepted as "safe", because thousands of electronics workers have not shown a syndrome. Some people have specific reaction to rosin smoke and should just avoid it. Water-Wash solder is something else. I see no need for it in tube-amp construction. Flux (electronics-grade) MAY remain on most assemblies and do zero harm. The exception is very high impedance (DC) PCBs in damp climates; also some SMD work which is too small to hand-solder (with common techniques), these may have to be de-fluxed. Rosin comes off great with harsh Freons, poorly with alcohol, turpentine may soften it but turp residue is as bad as the flux (worse because it has not been tested for electronics and may have mystery adulterants). There are other solvents, all illegal to dump in sewers or fields. WW flux comes off in warm water and is safe in sewer systems.