Saxophone Construction

Metals
A standard saxophone body is made out of brass (most commonly), copper (the Buffet Prestige S3 and some Monique horns), bronze (some Yanagisawa models) or plastic (the Grafton Plastic Alto is the only example). There is sometimes a sterling silver bell added (the King Silver-Sonic and some Yanagisawas are examples) and any horn can have a standard neck made out of bronze, copper, brass, silver or gold.

It is generally accepted that the metal used in the construction of the body of the saxophone, and that of the bell, is secondary to the metal used in the neck. This is because the saxophone is an open-pipe instrument. While a saxophone will always take advantage of the neck in producing a note (i.e. air must always flow the neck), a saxophone will only take advantage of the sterling silver bell of your Silver-Sonic, for example, when you're playing low C#, C, B and Bb.

Copper and bronze are supposed to "warm up more evenly" and produce a more mellow tone. Gold has an extremely dark sound. Silver is very bright. Brass is kinda "in between". Plastic, I have been told, is very free blowing, but has a bit of a very bright edge.

(Nickle plating is very much like silver in its acoustic properties. Note that most nickle-plated horns were marketed as a company's "step up" model, with brass being their lowest and cheapest models.)

Plating or lacquer affects a saxophone's sound to a lesser extent and it is based on how much plating is actually used. The most common platings are nickle, silver and gold (which is actually gold plate over silver -- gold does not electroplate directly to brass). Lacquer or enamel is essentially just a cover for brass or silver and probably does not affect sound much. There are some rumors in the Selmer world that removing the lacquer from your horn does make your horn easier to blow, but I don't buy it.

In any event, a horn with a solid gold neck that has heavy gold plating (like an early Conn or Buescher) will have the darkest sound. A horn with a sterling silver neck and silver plated body -- and perhaps a sterling silver bell -- will have the brightest sound.

A heavy brass horn can sometimes produce a tone as dark as a gold plated one, provided the horn uses a lot of metal for it's construction (thick walled).   The prime examples would be the Selmer Mark VI and the older Martins.

Thin walled brass horns have a very tinny sound, with the Yamaha 23's being a notable exception (it's not as tinny as one would expect from the thickness of the metal) and the Holtons being the prime example.

The thickness of the metal is the primary reason why you don't want to relacquer a horn: the shop will dip the horn in a chemical solution to remove all existing lacquer and then buff the horn so the new lacquer will stick, and shaving off a bit of the metal in the process.  This is the reason you can see two identical looking Mark VI's next to each other, with a sign on one that says "relacquered" and the other saying "original lacquer" and the "original lacquer" will sell for about $500 more.

There is more than one study out there that "proves" that the material used in the construction of a conical bore instrument has no bearing on it's sound. In practical application, however, I can heartily disagree. In my experience, I have played horns with many different platings and have determined a wide variety in timbre. My wife, who hasn't played many models of saxophone, was able to hear and feel the difference between a Selmer Mark VII and her Selmer USA pro model.

The Body
A saxophone doesn't have to be a specific length.  It can be shorter, to a certian extent, if the bore (the horn's innards) is bigger.  This means that you've got to use a thinner interior wall to accomodate the bore. This makes a stubbier horn you can pump a lot of air through (Conns are like this).

Another thing to look at on the saxophone body is the placement of the bell's tone holes (see below).   If you're playing tenor, for example, and they aren't on the right side of the horn, the bell notes (C#, C, B and Bb) will be very stuffy.  The alternative for the alto, provide you play the horn between your legs instead of slinging it across your right side, like a tenor, is to get a straight model, like those designed by LA Sax or Buescher. Oddly, while straight altos and tenors should be easier to play, easier to manufacture and more technically sound, these horns have their own intonation problems and are not something I recommend (except for collecting Buescher Straight Altos :)

Tone Holes
The best article around is probably the one at CyberSax.  I just want to mention that it seems to me that the world's best "classical music" vintage instruments have soldered, straight tone holes (Buffet SDA, Selmers). Most of the famous "jazz" horns have rolled tone holes (Conn, SML).

Resonators
Resonators are the things on the pads that look like little plates.  They are made out of metal (flat or domed, snap or screw on) or nylon.  They are also called "tone boosters" because they do exactly that, acoustically, making the overall tone a little brighter and, as a side effect, make your keys seal better.  They are not something I would say all horns are required to have (and a lot don't), but they can help brighten the sound of a heavy walled horn (and the better pad seal is a good thing) and are a detriment to all thin walled horns, especially metal resonators (you don't want metal resos on a YAS-23).

The best example of resonators being put to good use, that I've seen, was the stock Buffet screw-in, domed metal resonators on the Super Dynaction.  The SDA is a horn about as heavy (or a little more so) than the Mark VI, so the slight brightness of the resonators, balanced by a classical mouthpiece and the horn's innate darkness, creates a very warm timbre.

Things That Don't Affect Tone Quality: Springs and Keys
You want to have decently springy springs.  The old needle springs (used up to the 60's) don't retain their springiness long enough, but have a very nice light feel.   The newer "blue steel" springs I find to be a little too stiff for my taste.  Some saxophones (like the Selmer Super 80's) have a spiral spring (like the kind found in your click pen) in the key rods in addition to the straight spring to make the action lighter. Some saxophones have screw-in, gold plated screws called "Norton" springs. These are extremely nice and are most famously found in the vintage Buescher horns (400 and Aristocrat models).

The keys are probably the most undervalued part of the horn, and they shouldn't be.   They should be made of a stong, non-bendable metal, should be "loose", so you don't have to mash the key down (like the low C#, B, Bb cluster on Conns and Bundys), shouldn't click when pressed (even if you have a Martin Typewriter :) and shouldn't come in contact with another key or rod when pressed (like it does on my Amati).  The keys should be an easy reach for your hand and should be logically layed out (not like the Holtons).  The best setup really is what's on the Selmers, from the Balanced action until today, with Yamaha coming in as a close second.