Technical Information -

The reasons for fitting a ResoBridge to a resonator guitar

In Brief -

All resonator guitars have fixed, straight string saddles. Any compensation for intonation must be built in to the guitar by choosing the distance set between the twelfth fret and the saddle and through setting the saddle at a slight angle to the strings. This does not allow for changes in string gauge or action and doesn't work well for open tunings, where the musical interval between the strings isn't as regular as standard tuning. The open tunings of C and D, often used for slide playing, require that the bottom string length should be significantly longer than the other strings.

Many vintage instruments and a lot of the Korean copies have poor intonation because the distance between the twelfth fret and the saddle is too small.

The ResoBridge S and ResoBridge T have moveable saddles and allow near perfect intonation, because you can set the individual string length just like on many electric guitars. The range of saddle movement allows compensation both for the large offsets required by open tunings and for the dimension problems found in some vintage instruments and in some modern copies.

Intonation - What is it and why does it matter

For musical instruments the term 'intonation' refers to the relationship between the notes to which an instrument is tuned. For reasons which we won't go into here, a full discussion of intonation (i.e. what's 'in tune') is quite a thorny subject. For now lets agree that we are talking about doing the best we can with equal temperament.

On a guitar each string can be tuned to any note within a limited range determined by the strings length, mass and tension. Pitch rises as strings get shorter, lighter or tighter and falls as strings get longer, heavier or slacker. When a musician plays, notes are selected principally through changing the vibrating string length by 'stopping' (pressing down) each string against the metal frets that divide the length of the instruments neck. In theory halving the string length should exactly double the note frequency compared to the note frequency produced by the full length of the string. Ideally there should be a difference in pitch of exactly one octave between the open note and the note when stopped at the twelfth fret. However this would assume a change only in string length and not in tension.

In the case of a real guitar, when strings are fretted at the twelfth fret (or in fact any fret) there is a change in both length AND tension because the strings are stretched above the frets and must be pushed down to make contact with them. Consequently the octave note at the twelfth fret tends to sound slightly sharp. Intonation compensation can be applied by increasing the vibrating string length, making the distance between the twelfth fret and the bridge slightly longer than that between the nut and the twelfth fret. This compensates for the increase in tension caused by fretting.

Although electric guitars often feature a complex bridge design with a moveable saddle for each string, allowing individual intonation adjustments, acoustic guitars normally have a single fixed saddle. Early guitar makers simply placed this saddle at right angles to the string. This scheme would work well if the string masses where chosen appropriately. However string manufacturers have tried gauging strings in this way and there is a problem; it results in a string set that isn't pleasant to play. So setting intonation for a practical string set requires that each saddle position is set for a different vibrating length for each of the six strings. Acoustic guitars are usually designed with a single saddle set at an angle that provides a compromise length of string across all six. Really modern designs include fixed offsets shaped into the saddle.

Some background on intonation and the resonator guitar

Intonation compensation is a relatively modern innovation for the guitar and seems to have been driven by the move to amplification and the modern use of the instrument as a lead voice. Under these circumstances any tuning deficiencies became more obvious than when the guitar was used purely acoustically for simple strummed accompaniment and rarely played above the seventh fret.

Even today people who should know better will tell you that if you are playing slide style then the instruments basic intonation doesn't matter (since the precise note is determined by the musicians placing of the slide rather the relative position of a fret). This ignores the fact that many slide players incorporate both fretted and slide notes into their playing and in any case guitars with good intonation just sound better.

The original National resonator guitars are an incredible invention and a unique product of the transition between small scale European craftsmanship and industrial mass production techniques that occurred in America in the 1930's. At the time the search for greater volume culminated in the amplified electric guitar and the resonator guitar was quickly over-shadowed. Partly because of their particular bridge construction and partly because it just wasn't common practice at the time, the original Nationals do not incorporate any method of intonation adjustment.

Fortunately, thanks almost entirely to individuals such as McGregor Gaines and Don Young of the National Reso-Phonic Guitar Co. and musicians like Bob Brozman, the resonator guitar has gained a new lease of life. The current models manufactured by National Reso-Phonic do make some effort to allow for intonation through changes from the original neck dimensions and on the tri-cones by a slight offset angle of the T bridge. However apart from some small movement in the bridge and cone position there is little opportunity for intonation adjustment.

In Bob Brozmans really excellent book 'The History and Artistry of National Resonator Instruments' on page 242 he says that for better intonation the strings should contact the saddle at the back edge (i.e. the edge nearest the tailpiece) and each string slot should be dressed away from this point to allow each string to vibrate freely. Because the maple saddle in resonator guitars is quite thick this makes quite a difference in vibrating string length. Although this may work in practice for some guitars (and certainly for some recent Korean copies that are sold with the string contacting the front of the bridge and whose intonation benefits greatly from this being changed to rear edge contact) National Reso-Phonic don't set their guitars this way, having dimensioned and set their necks to provide a suitable string length. National Reso-Phonic also say that they have been told by John Dopyera - inventor of the resonator - that it's not the way that the original National instruments left the factory. With old instruments it's possible that over the years with many string changes and even cone changes the original bridges were reversed, either in an attempt to improve intonation or just by mistake.

Resonator types and cone replacement

There are three types of resonator design used in resonator guitars - Tri-cone National, Single cone National and Dobro.

The Tri-cone design was developed by John Dopyera and was the first design built by the National String Instrument Corporation. A Tri-Cone resonator guitar has three spun aluminum cones set point up, with a T shaped, sand cast, aluminum bridge, resting on the points of the cones. The mass of the bridge and its consequent inertia helps to keep energy in the strings, rather than adsorbing it when they are plucked. This gives the Tri-cone resonator its singing sustain. Although having a high mass bridge with lots of inertia helps sustain, there is a trade off - since the energy stays in the strings it isn't transferred to the cones, so volume is potentially reduced. This effect is compensated for by the length of the bridge arms acting as magnifying levers, so that the twisting vibration imparted to the bridge by the strings is amplified and applied to the three metal cones. The cones act like loudspeaker cones and again amplify the sound by coupling the vibrations to the air around the guitar. The Tri-Cone is the most complex of the resonator guitar designs and is therefore the most expensive to manufacture.

About two years after the Tri-Cone went on the market the single cone National and the Dobro were developed as rival designs. This was due to a disagreement between George Beauchamp, one of the original partners in the National company, and John Dopyera about the best way to build a simpler and cheaper resonator guitar. John Dopyera and his brother eventually left National to set up their own company making the Dobro guitar - a wooden body guitar with a single large aluminum resonator. The difference in these two designs is that, in the single cone National the cone points up and is centre driven, in the Dobro design the folded cone points down (mostly) and is edge driven.

The National single cone is perhaps the simplest resonator guitar design possible. The nine and a half inch aluminum cone rests on a circular ledge extruded below the top of the guitar (sometimes the ledge is made from a separate piece and braised to the top). A circular wooden biscuit is glued, tacked or fixed by a centre screw to the point of the cone. A strip of maple runs across the biscuit and acts as a saddle for the strings. This simple design, because of its relatively lightweight bridge and direct coupling of the strings to the cone, produces a loud 'plunky' tone.

Fitting a ResoBridge will improve the sustain and tone of these guitars by increasing the mass of the bridge biscuit. The amount of added mass is carefully chosen so there is little effect on the overall volume.

The Dopyera brothers (the Dobro name comes from the Do of Dopyera and the bro of brothers - it also means 'Good' in Slavic) Dobro design abandons simplicity in favour of better tone and sustain. The cone is a little larger, at ten and a half inches and uses a more complex 'folded' design. It's mounted with most of the cone pointing downwards (the centre of a Dobro cone folds or reverses, to form a second inverted cone pointing back into the outer main cone), so that the cone shape naturally projects the sound out of the guitar. The maple or willow saddle strip sits at the middle of a large sand cast aluminum 'spider'. This spider has spokes, or legs, that rest on a ridge close to the edge of the Dobros cone. A tension screw runs through the centre of the bridge and down through the point of the inner cone. Once again John Dopyera used the mass and inertia of an aluminum casting to help improve his guitars sustain and any volume loss is compensated for through the lever magnification effect of the spider legs that drive the cone. Dobro style guitars are known for their good sustain and mellow tone, which is quite distinct from the other two resonator designs.

In principal replacing the cones on these guitars is actually quite simple, since they were designed so the cones could be replaced. It's simply a matter of removing the strings, unscrewing the screws that hold down the coverplate, lifting off the coverplate to expose the cone and then lifting out the cone itself. The bridge is removed from the old cone, fitted to the new one (or the bridge may be replaced) and the guitar re-assembled. Simple!! Well yes it is, but its a process that requires care, since the spun aluminum cone can easily be damaged and the bridge position and set-up must be carefully adjusted for action and intonation just like any other guitar.

Its often recommended that you only change strings on a resonator guitar one at a time. This is because the bridge and cone assembly is only held in place by string pressure. If you remove all the strings and are careless, the bridge and cone may move out of position and the guitar intonation may shift or you may even damage the cones when you re-string and bring all the strings up to full tension. Resonator cones also 'play in' and new cones gradually bed down in the sound well, taking time to produce their best tone. For new cones this process may take several weeks. Removing all the strings allows the cones to relax and when re-tensioned they will again take a little time, usually a couple of days, to produce their best sound.

When you're dealing with valuable vintage instruments any adjustments take on another dimension since they possibly have an effect on the instruments value. Cover plate screws may be rusted in or their threads may be stripped, bridges may be glued or tacked on to the cones and on an old instrument the various components may no longer fit well, leading to buzzes and rattles when they are re-assembled.

Replacement cones

Genuine National replacement cones can be ordered direct from National Resophonic. Otherwise 'no name' cones or Quarterman cones may be ordered from Stewart Macdonald (every cone I have ordered from Stewmac has arrived damaged due to very inadequate packing).

The cones fitted to cheaper Korean resonators can be extremely variable, although they look the part with the right spin marks and embossed spirals, some of them sound dreadful ! Recently cone quality seems to have improved and some of the Vintage brand Korean made Tri-cones I have tried sound very good (unfortuantely the Vintage resonator guiatrs are now made in China and the Chinese cones or 'domes' resemble hub caps in both appearance and sound). I have not tried the 'no name' cones supplied by Stewart Macdonald, but I have fitted Quarterman cones to single cone Korean guitars, resulting in a huge improvement in tone. Unfortunately the Quarterman tri-cone replacements I have tried sounded dreadful compared to the National originals. The Quarterman tri-cones seem to be spun from thicker material. Quarterman cones have a very high reputation as the absolute premium cone for Dobro style resonators. NOTE - I have recently changed my opinion of the Quarterman Tri-cones. I fitted a set I had to a Chinese made AMG2 Tri-cone as replacement for the truly dreadful Chinese hub-caps. Once given a good chance to play in the Quarterman cones are clearly far superior to the Chinese originals. The Quartermans do sound different to the National Resophonic cones, they are mellower and don't have as much bite as the Nationals.

Common resonator open tunings

Open G and open D are two very commonly used tunings for slide playing. Open G, also known as Spanish tuning is tuned D G D G B D (from bass to top string). To go into open G from standard guitar tuning, tune the top and bass string down one step to D then the fifth string down to G.

Open D is known as Vastapol or Sebastopol tuning since it was used for a seminal blues tune 'Sebastopol Blues'. For open D tune the top and bass string down to D, the third string half a step down to F# and the second string down one step to A. So you have D A D F# A D for open D.

Intonation adjustment and open tunings

Slide players commonly use 'open' tunings because they allow chords to be formed by simply laying the slide straight across the strings. Open tunings require a greater spread in vibrating string lengths between the six strings than 'standard' tuning. In particular the bottom string needs to be significantly longer than the other strings for good intonation. This just isn't possible using a single angled saddle. The ResoBridge designs allow for the required string offsets.

Intonation and the acoustic guitar

Intonation defined

Coming up with a concise, meaningful and accurate definition of the term ‘intonation’ is surprisingly difficult. Here is one definition -

Musical Intonation – The accuracy with which an instrumental or vocal tuning conforms to a theoretical tuning scheme.

The inherent problem in tuning schemes

Most guitars are built to conform to the equal tempered tuning scheme. This is a method of calculating the frequencies of notes in a 12 note scale so that it is possible to play in a wide variety of keys without the notes in any particular scale sounding too much out of tune. In practice the fret positions are calculated to conform to the 12 note equal tempered scale.

It is theoretically impossible to build an instrument based on a 12 note division of the octave that will produce perfect harmonic series in any key. It is possible to get closer to ‘perfect’ harmonic tuning by dividing the octave into more steps or notes. On a guitar this requires fitting more frets to the fretboard.

Why there is a problem with guitar intonation

On a guitar various notes are produced by pressing the strings down behind different frets and therefore changing the vibrating length of the string. The positions of the frets on most guitars are calculated by assuming that the strings vibrate in ‘perfect’ or simple fashion. For convenience in making the calculations it is assumed that the strings have no stiffness, so the vibrating string length is determined precisely by the points were each string bears against the nut and bridge. It is assumed that the string vibrates as though there were a perfect pivot or hinge at these ‘witness’ points. It is also assumed that string tension when tuned to pitch, remains constant. In practice with real strings none of these assumptions are true and string stiffness and tension change both have an effect on tuning.

String stiffness

It is impossible to make musical instrument strings without some degree of stiffness and the thicker the string the stiffer it tends to be. String designers simply do the best they can with the materials available to minimise string stiffness. The biggest breakthrough in this endeavour was the invention of the wound string. This meant that the bass strings could be made with a thin and therefore flexible core, with the added weight of the winding causing the wound string to vibrate at a lower pitch. The winding has relatively little effect on the stiffness of the string. String stiffness effects tuning because, when the string vibrates, it acts as though it were shorter than the distance between the witness points at the nut and saddle.

String tension changes

Because the strings are tensioned above the fretboard and have to be pushed down to contact the frets the act of ‘fretting’ a note increases the tension in the string and sharpens the pitch of the fretted note. The degree of sharpening depends on the open tension of the string, the construction of the string, the height of the string above the fretboard and the chosen fret or note being played. Tension even changes when the string is plucked or picked. It increases slightly at the very beginning of each note and then falls as the note decays. The louder the note the higher the tension change, therefore the sharper the note and the greater the pitch change during the decay.

Intonation compensation

The intonation inaccuracies due to string stiffness and tension variation can be compensated for by small adjustments in the length of the strings. This is known as intonation compensation. However this compensation is applied it is always only making the guitar conform more accurately to equal tempered tuning. Equal tempered tuning of itself is imperfect when compared to the pure harmonic series.

Simple intonation compensation

The simplest form of compensation is applied at the bridge of the guitar. Electric guitar bridges are often fitted with a moveable saddle for each string so that compensation can be applied individually to each string. On acoustic guitars the saddle is simply fitted so the vibrating string length is slightly longer than theory dictates for a ‘perfect’ string. The saddle is also set a slight angle to allow for the different amount of compensation required across the six strings. Some acoustic guitars have a split saddle where the top two plain strings are compensated separately from the wound strings. Intonation on most acoustic guitars cannot be adjusted easily. The position of the bridge and saddle are fixed during design and construction and are only going to produce the best intonation for standard tuning with one particular string set and action height. Poor intonation on an acoustic guitar is most often due to a very high action. A high action is often the result of string tension bowing the neck forwards, moving the neck set in relation to the body and lifting or bellying the bridge and soundboard upwards. Small amounts of forward neck bow can be reduced by adjusting the truss rod. Large amounts of neck bow and neck tilt can only be removed by an experience luthier.

Soundboard ‘belly’ distortion correction

Soundboard bellying can be corrected by fitting a JLD Bridge System (also known as the bridge doctor) from JLD Guitar Research (www.jldguitar.net). The bridge doctor is an adjustable retro-fit bracing device that is easily fitted inside the guitar body. It allows a variable amount of pressure, adjusted by a hex screw, to be applied to the guitars bridge, via a wooden dowel bearing against the centre of the end block. This rotates the bridge and bridge plate, in opposition to the tension from the strings and consequently flattens any behind-the-bridge bellying in the soundboard. In addition to correction any soundboard distortion, the JLD Bridge System (or Doctor) usually improves the sound and volume of a guitar.
The JLD bridge doctor is fitted as standard by the Breedlove Guitar Company and is routinely used by Taylor Guitars to rectify belly problems.

More advanced intonation compensation systems

More accurate compensation for the characteristics or real strings can be applied by compensating both the nut and the saddle.

There is one fairly well known compensation system of this type which is marketed with a large degree of hype. The person behind it, who claims to have invented the principles of nut and saddle compensation, has patented his ideas and sells a license for its use to guitar companies and for after market modifications to guitar repair shops. The American patent for this system is so poorly written and full of half-baked nonsense it is amazing that it was ever granted. Although this system does offer a degree of improvement there are other methods and products based on a better understanding of the principles involved. In this system the nut position is moved towards the bridge by a calculated amount and then a series of small tuning offset are applied to each string.

A better and more thoroughly understood system of nut and saddle compensation has been developed by luthier Gregory Byers (www.byersguitars.com).
Some pioneering work on this subject was done by the Bartolini brothers and their work has been further developed by Byers. Earvana (www.earvana.com) make a compensated nut and complementary saddle based on the principles developed by Byers.

In conclusion

Guitars have the equal temperament tuning system built in to the spacing of their frets. This tuning system is a compromise to allow musicians to play in any key. Since it is a compromised tuning system, intervals for any particular key will never sound as pure as the true harmonic series. However the perceived tuning accuracy, or intonation, of most guitars can be improved by adjusting them to conform to equal temperament intonation as accurately as possible.
The string gauge, string construction, action height and open string tuning pitch used for each string all have an effect on the intonation. Changing any or all of these, once accurate intonation adjustments have been made will mean that re-adjustments must be made.

Terry Relph-Knight © March 2006