A method for decreasing detune due to cabinet flex

Jim Pitman · Post by **Jim Pitman** » 10 Sep 2010 6:46 am

At the risk of beating a topic to death.....
PSG Cabinet detune – the effect observed whereby raising the pitch of one or more strings drops the pitch slightly on those strings not raised.
I noticed a while ago while working with a steel guitar maker that cabinet detune gets worse in proportion to the number of strings a guitar has.
I'm talking single necks on like body here only for the time being. That is, a 10 string guitar has a small amount of detune, a 12 string guitar on the same cabinet has more detune, and a 14 string guitar on same cabinet has even more. It just so happened there were three like bodied guitars in one place at the same time, a single 10, single 12 (mine) and a new single 14. At the time we had access to a fine movement measuring device and determined the following. Detune is mainly a result of the top body wood flexing. Yes, the gauge of the string and whether it is wound or not makes it more or less sensitive. Yes, the amount of pedal pressure one mashes the pedals with affects the detune, (the best argument for raise helper springs) However, I believe the primary cause is: the sum of the collective tension that the group of strings apply to the top side in relation to the stiffness of the top wood is responsible for the degree of detune.
The maker had also just built a couple of guitars with high quality jig board plywood rather than maple and these guitars had exhibited more detune than the maple ones. Tests with the position gauge and weights confirmed that the ply did indeed flex more than the maple when a weight was place in the middle with its sides supported.
This is also supported buy another casual observation I made. I had owned an earlier vintage PSG that was a thick bodied guitar. The wooden neck is one inch thick and the top wood is also one inch thick on this design. This guitar had very little detune. Unfortunately, I never liked the tone of this particular guitar.
Since the all the string tension is on one side of the top wood, and the strings sit an average of 1.25" above the top, the top flexes or bows. My guitar as well as many newer brands uses a 0.625" thick top. I theorize that a thinner top makes for a louder better sounding guitar (Emmons for example). I really like the tone of my guitar but it does detune more than I would prefer.
Here's my hypothesis: Once the top of a PSG is bowed it gets progressively easier to bow it further by increasing the tension of a string. This accounts for the behavior noted above. The bow in relationship to the compression force is not a linear function but rather is logarithmic until finally the piece breaks.
I mentioned this to a mechanical engineer and he said my theory is supported by Euler's buckling theorem. Euler was a mathematician in the 1700s who described stress affects with formulas.
So how do we reduce top wood flexing?
One possibility is to lower the strings. I saw a home made PSG with no neck and the strings were only 3/8” off the body. This guitar had very little detune but it was ugly as sin and didn’t sound very good to boot.
The method I would like to try is to incorporate a truss rod so to speak. That is, attach a metal rod that runs from end to end on the underside of the PSG with a turnbuckle in the middle. Perhaps it would be integral with the key head and changer some how such that one could pull the buckle out of the top of a fully strung guitar by turning the turnbuckle. According Euler’s buckling theorm, once the top is straight it would take a very high force to start it bending again. This force would be more than a single or dual string pitch raise could apply.
There may be reasons this is impractical. We are talking very fine movement here.
Temperature and humidity may require we constantly adjust the tension.
How do you know how much tension to apply?
Will it affect the tone?
More wieght and complexity - maybe not be worth it.
Any thoughts?

richard burton · Post by **richard burton** » 10 Sep 2010 7:03 am

What if the body was built with a deliberately induced bow, but in the opposite direction, so that the tension of the strings would bring the body almost (but not quite) back to straight ?

Then there would be no logarithmic effect to amplify the cabinet drop.

Steve Perry · Post by **Steve Perry** » 10 Sep 2010 7:12 am

Do the aprons come into play in this or is it strictly the top wood? My thought is that there are acoustic instrument makers that use a wood-carbon fiber laminate to add stiffness to necks and bracing. Possibly an apron made of cf between two pieces of wood would lessen the drop.

Jim Pitman · Post by **Jim Pitman** » 10 Sep 2010 7:34 am

Excellent idea Richard! If you put a slight radius in the front and back apron then cap it with the top. I think you are onto something.

Stephen I believe the apron does come into play. It does stiffen the top. It's likely the sides bow out perpendicular to the bowing mentioned. In fact a mechanical engineer suggested that the bottom of the guitar ought to be closed off to increase stiffness.

Rich Peterson · Post by **Rich Peterson** » 10 Sep 2010 8:55 am

I'm surprised carbon fiber reinforcement hasn't become the standard by now. It's the first thing I would ask for in a "dream PSG."

Storm Rosson · Post by **Storm Rosson** » 10 Sep 2010 10:09 am

In a way previous post ....last year ,I posted some pics of a BMI S-10 basket case I ressurrected and used the preloading the body by using a LARGE coupla pipe clamps to pull in the bottom of the psg (all Al frame),then tightly fit the deck into the frame, shimmed at the ends of the deck/frame to insure tightness then removed the clamps (the body was also cold welded with epoxy when reassembled).....that old rough rebuild has virtually no cabinet drop (less than 1/2% according to a Peterson strobe, and sounds real nice too (GeorgeL E-66 pu), and stays in tune better than most pianos. So I think the pre-loading and the truss rod are both viable solutions.I have been working on a design that incorporates 2 truss rods, one at/in the bottom of both aprons with tension adjustments at the bottom on each side of the tuner head endplate....haven't made a prototype yet but it should work....imho...Stormy

Bob Hamilton · Post by **Bob Hamilton** » 10 Sep 2010 11:07 am

It works Jim. I have been installing 1/8" rods with turnbuckles on my guitars to counteract cabinet drop for years. It generally cuts the cabinet drop by half or more. A couple of D-10 guitars I have rebuilt were not glued between the necks, just screwed together on 1" centers under the guitar, and there was significant cabinet drop. I built a jig before re-gluing them that added upward camber (preload) to the guitar and it took out the other half of the problem. I also preloaded the front apron rails. It all helps..

Storm Rosson · Post by **Storm Rosson** » 10 Sep 2010 11:28 am

Thx Bob that validates my ideas I think, glad to hear about the truss rods working. How/where are your's placed?...Stormy

Jerry Overstreet · Post by **Jerry Overstreet** » 10 Sep 2010 12:08 pm

I'm curious to know how much less deflection there is on a keyless 12 or 14. It has been proven by Sierra that there is less detuning when reducing the length of the string pull behind the nut.

Have you done any comparisons with keyless guitars or would you surmise that the detuning would multiply cumulatively as more strings are added, regardless of keyhead design?

How about overall cabinet length?...or detuning due to tension applied to the changer axle...particularly when pedals are activated?

Bob Hamilton · Post by **Bob Hamilton** » 10 Sep 2010 2:03 pm

Hey Storm. I try to get as high on the endplates as I can to anchor each end of the rod (close to the strings), and generally go over a knee lever bracket or use a spacer near the center of the cabinet to direct the force generated by the tightening the of the turnbuckle. The greater the difference in distance, and the tighter the rod, the more upward force is generated on the cabinet. Every guitar is different. It's actually sort of a truss, I guess. I think of it as an external truss rod, as you would find in a six string guitar neck. I have toyed with the idea of placing a truss rod in the front apron, but I think you can do better by preloading the front apron rails. BH

Bob Hamilton · Post by **Bob Hamilton** » 10 Sep 2010 2:08 pm

I started using this method to straighten out a cabinet that was water damaged and had been left to dry while still strung. It had about 1/4" of bow that I wanted out, and this method worked really well to straighten out the guitar, so I just left it on there. It took a few months of reverse tension to straighten it out. Zero drop now. Bob

John Billings · Post by **John Billings** » 10 Sep 2010 2:49 pm

Why not just use a couple of Fender type trussrods? Adjustable from a couple of holes at one endplate or the other. Embedded into the underside of the top. No fugly turnbuckles or extra rods under the body.

Storm Rosson · Post by **Storm Rosson** » 10 Sep 2010 3:22 pm

John if u used straight trussess in the deck u would loose most of the mechanical (lever) advantage needed. What Bob has just out lined is more like a suspension or truss bridge, with the center upright transferring the tension created by tightening the truss rods to the center of the guitar deck from the bottom. Thus directing the counter tension to the place it is most needed and effective. I have been toying with a similar Idea as a retrofit for wood body guitars that might be plagued with a bad drop prob......Bob's method sounds economical and easy to get parts and install on most any guitar tho, nice application Bob, simple and effective....Stormy

Chris Lucker · Post by **Chris Lucker** » 10 Sep 2010 3:31 pm

I think the reason that 12 string guitars have more cabinet flex than 10 string guitars is because the cabinets are longer. I could be wrong.

From my experience, cabinet drop is about pedal pressure and improperly set up guitars. Helper springs are not the answer. Add a third adjustable stub of a leg in the center of the pedal rack to the floor -- a self leveling device. Or use a block of wood, or a phone book.

Have you noticed that Day tuning has more cabinet drop than Emmons on the same guitar? Could it be that the A and B are closer to the center of the cabinet?

John Billings · Post by **John Billings** » 10 Sep 2010 3:44 pm

Storm,
Fender style trussrods are not straight. If they were actually straight, they would have almost no effect in countering string pull. I think they would work.
http://en.wikipedia.org/wiki/Truss_rod

Storm Rosson · Post by **Storm Rosson** » 10 Sep 2010 4:01 pm

You're rite about that John, I'm still not sure they would be effective in a psg deck but hey if nobody's done it then we'll never know for sure ....hint, hint.....Stormy..

Chris Lucker · Post by **Chris Lucker** » 10 Sep 2010 4:10 pm

I have the first pedal steel that Jerry Blanton built. It has a pair of quarter inch tool steel rods running from the changer end to the nut on the underside of both neck castings. These rods have nuts at each end to create tension. I asked Jerry about their purpose, and he stated that they were supposed to help control the expansion of the cast aluminum one piece neck, keyhead changer bridge. They may have an effect on cabinet drop as well, but I don't know if cabinet drop was invented yet in 1962 or 1964 whenever jerry made the guitar.
To clarify, the rods are inside the hollowed neck casting, not under the cabinet.

Jonathan Cullifer · Post by **Jonathan Cullifer** » 10 Sep 2010 9:55 pm

I would imagine that the attachment points for the pedal rods would have something to do with it as well. In order for the top deck to flex where it meets the front apron, it would take much more force that, say, the middle of the cabinet. Also, I would imagine that the front apron bows outward in the middle.

I have 2 things I would like to try. If you move the attachment point on the pedals closer to the foot (or lengthen the pedal itself), you won't amplify the force of the foot as much.

The other idea is to have a "Floating" pedal stop bar. If the pedal stop bar on the front apron is only anchored on the ends of the guitar, near the legs (where the cabinet is strongest), there will be a tendency for the pedal stop bar to flex instead of the body. That way, the only force that the middle of the body has to take is the force required to depress it, but not take the force of the pedal stop.

Brainstorm as you like...

Chris Lucker · Post by **Chris Lucker** » 10 Sep 2010 10:18 pm

Make it simpler. Make your pedal stop pressure upward like on a push pull -- not downward.
But still -- stop the pedal rack from bending downward from heavy feet and you will stop the cabinet from following.

Jim Pitman · Post by **Jim Pitman** » 11 Sep 2010 3:58 am

Before you get too concerned with pedal pressure bending the cabinet try this. Actuate a raise with your hand underneath the guitar by displacing a puller to the left, (no downward pressure on cabinet). On my guitar I still get close to the same drop.

Storm Rosson · Post by **Storm Rosson** » 11 Sep 2010 6:44 am

Bob, the more I think about it ,the more I like your method. I think it could be a winner all around.Maybe some peeps could mull over ideas for materials, mainly pertaining to the "tension rod" ,we need a alloy that is workable and has high tensile strenght and low ductile movement when stretch force is applied. Now if someone with metallurgical knowledge could throw some ideas for this in here ,we could have a plan coming together...ok metallurgist needed please drop in ....>here<....Stormy

Johnny Thomasson · Post by **Johnny Thomasson** » 11 Sep 2010 7:09 am

I think Richard Burton's idea has merit. In engineering terms this is known as "pre-stressed" or "pre-loaded". It works on the same principle as an arch in building construction. It may not completely eliminate bowing, but a pre-stressed cabinet couldn't possibly bow to the same extent as a flat piece of wood (or other material). I would think the amount of "counter bow" necessary would not be noticeable unless compared to a true straight edge.

Unfortunately, this wouldn't be "retrofit-able" as would the truss rod idea. Maybe a builder will experiment with the concept. It would certainly be simpler and cheaper than the use of compensators as a means to minimize detuning as a result of cabinet drop.

Storm Rosson · Post by **Storm Rosson** » 11 Sep 2010 7:32 am

Yepper I think the preloading is a good idea as I used it before and got decent results. Bob's method by building a jig to to incorporate a slight bend during body construction sounds like the way to do that, but I still think an adjustable tension rod(s) to fine tune out the drop is a sterling idea to be used in conjunction with preloading...imo...Stormy

Tony Glassman · Post by **Tony Glassman** » 11 Sep 2010 8:05 am

I agree w/ an adjustable pre-loading truss rod running along the bottom of the deck from end plate to end plate, which would offset cabinet drop forces......or we could add "flying butresses"

Correct me if I'm wrong, but wouldn't a pre-loaded (bowed) deck decrease the surface contact between the deck and necks and screw up vibration transmission.

Some people feel the pitch drop is due to changer axle flex rather than cabinet drop. It would be interesting to set up the same guitar w/ A + B changes on pedals and on knee levers and measure the pitch drop engaging both seperately. You'd have to apply equal pressure in all trials.

If the an open string drops pitch in BOTH cases that would imply axle flex. If it drops only with pedal mashing, cabinet flex would seem to be the culprit.

Storm Rosson · Post by **Storm Rosson** » 11 Sep 2010 9:10 am

Not sure Tony but I think any tension whether from fp or kl would impart a force on the deck/body resulting in a bowing effect, however it would be interesting to note the diff between the two....Stormy

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A method for decreasing detune due to cabinet flex

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