by Seth Lee Jones, owner/operator of SLJ Guitars

After working on thousands of instruments as a professional Luthier, I have gathered together these facts to best explain why and how the two types of adjustable truss rods work, and their effects on the instrument’s overall tone.  I hope to shed some light on one of the myths associated with building a great sounding guitar.

Most fretted instruments have some neck reinforcement to keep the neck straight under the load of string tension. A hundred years ago, that usually meant hardwood lamination. Martin guitars had square ebony reinforcement inlaid under the fingerboard. Gibson used a triangular cross section insert of maple in the same position. When Martin lengthened their necks to 14 frets clear of the body, they started using a steel rod for reinforcement.

The first adjustable truss rod was patented by a Gibson employee, and was a standard feature on Gibson instruments starting in 1922. Adjustable rods of this same design are still in use today because they are very effective, light and simple. Called compression truss rods, they have no moving parts, and consist only of a steel rod with one threaded end.

The basic action of the standard compression truss rod is compression of the back of the neck. When the nut is tightened, the back of the neck is compressed and causes the neck to bend backward. It is really quite predictable because the back of the neck has a much smaller cross-sectional area, so it compresses much easier than the front.

A dual action truss rod is a more modern design and it is currently being used by some luthiers in lieu of the vintage single truss rod. The dual action rod is installed in a straight channel in the wood as opposed to the curved channel used by vintage rods. When adjusted, it exerts force on the neck in both directions simultaneously, pushing towards the fretboard in the middle of the neck and away from the fretboard at the nut and heel.

A neck with a dual action rod installed is often more stable and less influenced by climate changes. However, some players notice the dual action truss system has an adverse effect on the tone of the instrument, due to the weight of the second rod and the additional wood removal required for installation.

Since dual action rods have moving parts they are also more prone to vibrate sympathetically with frequencies produced by the strings. To combat this problem called “sympathetic resonance” it is common practice to encase these types of truss rods in silicone or shrink tube material. This effectively dampens the resonance of both the truss rod and, unfortunately, the neck.

Resonance is the natural frequency of a component or combination of components. All structures have a resonant frequency. If you impact the structure with enough force to make it move, it will vibrate briefly at its natural frequency. Resonance serves to amplify the vibration due to whatever vibration force is present at (or near) that resonant frequency. It is important to note that resonance does not cause vibration – it amplifies it. Resonance can cause annoying buzzes in a concert hall if pieces of equipment such as fans fixed to the ceiling resonate when a note is played by the orchestra.

In the case of a dual action truss rod equipped neck they can be susceptible to sympathetic resonance, wolftones, or dead notes on the fretboard. The classic example is demonstrated with two similar tuning-forks of which one is mounted on a wooden box. If the other one is struck and then placed on the box, then muted, the un-struck mounted fork will be heard. In similar fashion, strings will respond to the external vibrations of a tuning-fork when sufficient harmonic relations exist between the respective vibratory modes. A unison or octave will provoke the largest response as there is maximum likeness in vibratory motion.

The original compression truss rod is less susceptible to sympathetic resonance since they don’t contain multiple parts. These necks generally resonate evenly throughout the range of their frequencies since they are not dampened by mechanical resonance canceling materials, and energy is not absorbed by vibrating parts like multiple truss rods.  An object free to vibrate, like a neck without moving parts inside, tends to do so at a specific rate called the natural resonant frequency.

Collectors and players both value vintage instruments from the 20’s through 60’s for their superior tone. These vintage instruments all share one common factor. Fender, Gibson, Grestch, and Martin all had reinforcement rods of some kind that contained no moving parts. These simple reinforcement rods allowed these guitars to vibrate at their natural resonant frequencies rather than wasting the strings vibratory energy on parts that vibrate sympathetically inside the neck. These vintage guitars also had no dampening materials inside their necks that would absorb resonance.

In conclusion, there are several things at work in a great sounding guitar, and one of the commonly overlooked factors is the resonance of the neck. The type of neck reinforcement can drastically affect the tone and projection of a guitar. It can be the difference between a mediocre sounding guitar with dead notes and wolftones, and a wonderfully responsive full sounding guitar that has even note response across all frequencies . It is because of these facts that I’ve laid out in this article that I make my necks with the compression truss rod.

Here’s Billy’s prized vintage Gibson on the bench for a long overdue fret job. I’ll also be addressing his tuners, nut, and bridge.

Can you tell Seth’s been busy? We’ve barely had time to update the website lately with all of the repairs, restorations and custom builds going on in the shop. So what’s new? A lot!

Seth was recently featured in TulsaPeople Magazine for their January 2013 issue, which was a huge accomplishment for Seth after two years of nothing but hard work and dedication to giving Tulsa the best guitar repair and custom builds he can possibly do. Constantly learning and perfecting his craft, Seth genuinely wants to provide the best results to customers and it shows with this wonderful article that showcases his craft.

We’ve had some exciting repairs come through the shop, including blues legend Steve Pryor, who had his guitar in for a complete facelift including a great refret to keep his sound as true as possible.

Custom builds are abundant, including a recent Golden Ivy colored fretless bass project that had bassists in Tulsa reconsidering frets. Check it out:

We’re really excited about 2013 and all it has to offer as Seth continues to build a relationship with his customers in Tulsa and all over the world (even Kuwait for a recent headstock repair). Thank you for supporting your local Tulsa luthier and supporting certified craftsmen like Seth who take pride in handmade integrity, hard work and a true dedication to this wonderful art.

Have questions about SLJ Guitars? Feel free to contact Seth any time, and remember, quotes are always free. SLJ Guitars believes in giving accurate quotes and not wasting the customer’s time or money, so you’ll always get the most honest opinion in town at our shop.

Josh Tovar’s 72 Tele came to me from Nashville with a back-bowed neck and a nasty chisel hole underneath the pickguard that destroyed the end of the neck pocket. I used proprietary methods to repair the backbow, to get the adjustment range back and refretted the guitar, gave it a handmade bone nut, and grafted in a new piece of ash, re-routing the body in the original style.

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This custom tele-style build was handmade for local guitarist Cody Jefferson. Every aspect was hand carved to meet Cody’s needs as a player.

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This Gibson LG was restored for Josh Yarborough.

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1928 National restoration project for Jacob Tovar. This delightful find was discovered in a closet!

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This late 60s Gibson Flying V came to me with no headstock attached and no headstock for reference. Starting from scratch hand carving a new headstock form, the new headstock was grafted on to the body and the neck given a full lacquer paint restoration. The original serial numbers were also included for full period accuracy.

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