Re: I have a 1960s replica wire tune-o-matic but it lacks sustain
Physics dictates (thermodynamics) that energy degrades, moving from one form to another until it reaches the point it can't become any lower form of energy. In doing this, energy tends to follow the path of least resistance on its way to becoming weaker forms of energy. In a guitar, the string's vibration forms an exit for energy, and is the easiest exit for that energy up to a point - once you put more energy into the string's vibration than the string itself will be able to comfortably dispose of, that energy will find another way out. Increasing the sustain of a string relies on providing that energy with as few alternative ways out as possible.
As a result, things that will increase sustain -
1 - Harder contact points with less contact area. (Saddle, nut, tuner, frets, etc) The less surface area of the string is in contact with the hardware, the less area there is for energy to be transferred into the rest of the guitar. Making these areas harder will stop the string wearing into the hardware over time and lessening the efficacy.
2 - Less contact points.
The extreme case for this would be to have a guitar moulded out of an infinitely hard, dense, and strong material, all as one piece with zero adjustable hardware and therefore no moving parts beyond the string. Being infinitely impervious to any outside force, the string would be completely unable to dump excess energy into the structure itself, therefore sound, and a small amount of heat causes by the friction of the molecules and windings within the string itself, would be the only things that the string could produce to rid itself of kinetic energy and return to a state of equilibrium.
Seeing as we can't do this, things like making one piece moulded bridges without intonation adjustment, will have to do. This is one of the main reasons floyd rose guitars have common complaints about sustain - the number of components the string is attached to, all of which can move and be vibrated, is insane!
See also - why neck thru guitars sustain a long time, why fixed bridges sustain longer than tremolos, why locking tuners can help increase sustain (less movement around the capstan), things like Gotoh's magnum lock tuners are similar to this too, removing captsan post vibration from the system. etc etc.
This is the easiest point to actually address practically by design, so there's no shortage of products to help with it. Except maybe...
3 - More mass.
The greater the mass of an object, the greater its inertia and the greater the force required to put it into motion. In a guitar body, the inertia of the body vastly exceeds that of the string. If we continue to increase the mass of the body (presuming the body forms one homogenous system of equal strength, where the string is the only moving part), then the inertia required to move not only the body, but any part of the body, including resonance and pure vibration, becomes harder too. As a result, the string dumps that energy into sound once again.
This is why using brass hardware, tungsten sustain blocks, thick necks, thick bodies, heavy mahogany, and products like the "fat finger" all work to increase sustain - They add mass to the body. This also has the positive effect of moving "wolf tones" out of reasonable ranges - a very light, flimsy, weak guitar, will have wolf tones all over because it's essentially hyperactive - unable to supress the mixing of unrelated frequencies that will unintendedly boost the fundamental of a given note via waveform cancellation and addition. Making the guitar and its parts too solid, heavy and inert to have this happen, means they can only happen when huge amounts of force are used to strike the string. In an ideal world, such force that would never be used.
So yes, heavier hardware works to increase sustain. It's pretty much the only thing that DOES work, save for getting creative design wise and using hardware that suppresses vibration and cuts down on moving parts.
Oh, and you'll also get more treble lift - the treble frequencies, being the smallest amplitude, tend to be the ones that an inefficient system will bleed off first and fastest. (Hence why a bad guitar will have sharp sounding attack and ****ty dull sound notes that fade out too fast) - This is a good thing. Natural production of sound is a subtractive process - you can't add more treble to the string or the system. You can only design a system where less of that treble is lost. Since it's being lost because it's easy to bleed off via inefficiencies, making the system of sustain more efficient, will also mean your notes retain treble frequencies in greater quantity for greater durations.