PSG body

[richard37066]

Bent -

Ya got me 'cited about this ole stuff so I broke out the books and calculator again and did some numbers. This will most likely be a long one.

For the sake of simplicity I used 30" as the length of the members for the calculations. This is only representative and not to be construed as a typical length for any part of the guitar.

I chose a piece of aluminum bar as my "standard", knowing that it wouldn't bend very much under load. I treated the members as simple beams supported at the ends since the degree of fixation at the ends within the guitar is difficult to ascertain.

With a five-pound load, a piece of maple 3/4" X 3" X 30" would bend more than 3 times as much as an aluminum bar with dimensions of 1/4" X 3" X 30". Further, if you layed that maple bar flat (3" dimension horizontally) It would bend almost 25 times as much as the same maple piece standing vertically. This is the kind of thinking that went into my response to Bay regarding his question. We can deduce at least one thing here. If we glued the maple pieces together such that we simulated an apron attached to a top piece, then the apron would supply most of the strength, it being, at very least, several times stronger than the top. Nonetheless, WITHOUT an adequate aluminum side plate into which the crossrods are fixed, you can bet that the apron/top assembly - as stout as they may be - will contribute to cabinet drop if you mash a couple of pedals to the floor. How much? Don't know. Would you consider replacing any existing aluminum side piece with something much stronger - assuming that there is a piece in place already?

Another thing occurs to me and I may well be way off base here. For all intents and purposes - in the context of a PSG - the aluminum is not compressible. The wood is. Given that the top is already much weaker than the sides (aprons), if you punch a great big hole in it to accomodate the changer then you've only exacerbated the problem. Further, the wood being "compressible" to a degree, one might ask the question whether you have two things in concert which may contribute to "cabinet drop". The first is obviously the weakened top - the top may bend under pressure. The second may be that the changer is "rocking" on the surface of the top - the wood tending to spring back to its' former shape. Wouldn't take much "rocking" to alter the tuning by several cents. One would think that, after a while, the changer would make an indentation in the top and the "rocking" would only get worse. This doesn't seem to be the case unless the changer assembly is visibly loose.

Two things come to mind, now. The first is the stout bar placed from apron to apron in some Emmons guitars. Others, maybe - don't know. When the cabinet bends, the sides might very well splay in or out since the cabinet - mainly the front apron - is now flexed in some screwy manner and the apron is much weaker in that sideways bending mode. The added bar might well serve to tend to keep the aprons in a fixed and vertical bending mode. If I had an old instrument with horrendous "cabinet drop" then the first thing I'd do is to get a piece of good old cold-rolled and make myself a bar and screw it in place right in the center. Might help, might not. Worth a try.

The second thought that comes to mind is that damnable hole in the top which accomodates the changer. I would be sorely tempted to shore up that hole with a large piece of metal so as to spread the load out over a much broader area. How big a piece? your guess is as good as mine. I'd be tempted to make it as big as the hardware would allow even if I had to pull a fixture or two and then drill holes in the metal so as to remount them. To refer to your description - what good does it do to have 1/2" Grade 8 bolts 6" long if the top is going to bend?

Almost as an afterthought - what is the neck made of on your instrument? I know, there have been some on the SGF who have said that by loosening the screws that hold the neck in place that the "tone" of the instrument suddenly becomes world-class. That claim may or may not have merit. What is undeniable is the fact that, if the neck is firmly screwed to the top, then the top must, of course, be stiffened. But around the changer hole? Where are the mounting screws located? I'd be tempted to make that whole end of the guitar one giant sandwich so that NOTHING moves! Needless to say, you'd have to weigh that against the possibility of reduced resale value. Your call.

As far as I'm concerned, your thinking is on the money. The only question that remains is how to implement a fix. Now that you've got that miller cranked up and you're getting expert at making fancy gadgetry you should be able to quickly come up with a solution or two. I definitely WOULD be interested in your findings after you've made a modification or two.

I may not have helped you, but I sure am enjoying the conversations. That numerical example, above, was done solely to give you an idea of how much different pieces may bend under pressure.

Respectfully,

Richard




















x

[Georg]

Bend this...

...sorry, couldn't resist

Bent knows this already, but for the rest of you: those 4 bolts don't touch the top-plate and the changer-hole through the top-plate is so big that the aluminum on top barely covers it. That PSG is an anomaly.

But, if those 4 bolts were holding the changer to the top-plate the result would be the same: no body-drop. That's because the changer and neck is one piece so the neck steady the changer no matter how and where it is bolted. Would have to bend the neck itself to cause deviation, and aluminum doesn't bend much - especially when it is laminated and has steel reinforcement where it counts.


On regular PSGs the changer-mount should be as large (wide) as possible lengthwise, to spread the load and thereby prevent bending. Better to have a large changer-mount and a relative thin top-plate than a small changer-mount and a thick top-plate, as a large and naturally enough a bit heavy changer-mount will increase sustain and interact well with a thin sound-board/top-plate (my Dekley's sound-board/top-plate is very thin, and vibrates nicely).

Some of the old PSGs they say have excellent tone and low body-drop, seem to have quite large changer-mounts. Don't know much about their top-plate thickness, but with large/wide enough changer-mounts to spread the load over the top-plate thickness doesn't matter all that much for body-drop.

There is of course the possibility to insert a large, rigid, metal profile inside the body under the top-plate and changer to spread the load on lengthwise. That is better than increasing thickness of the top-plate, as there is no way to make wood strong/rigid enough around the changer hole and still get a reasonable good body-tone in/through that area, IMO. Besides: what's inside doesn't show so aesthetics may rule on to top-side


Since I'm not gonna have a top-plate in the normal sense on my own design/construction, I should rather not say more on the subject. Have to add that I prefer relative thin laminated wood/metal over solid wood of any type and thickness in PSGs both for tone, strength and weight though

[richard37066]

Georg -

Don't resist!

How old did you say that instrument was? Sorta makes ya wonder how a good design can fall by the wayside in favor of something else which proves to be inferior in its' performance.

That Dekley design should give folks a little food for thought.

Richard

[Georg]

Richard,
My Dekley was bought new in 1981, and originally it had too much body-drop because of small (narrow) changer-mount screwed onto a thin top-plate/sound-board. I am extremely sensitive to audible detuning, so that was no good. It picked up a good body-tone and had reasonable good sustain up to around fret 14 though.

I finished the modification seen on the picture in 1993, and the result was/is inaudible body-drop (at most measured to be less than 2 cent on regular low E string, less than 1 cent on all other strings). Picks up a good and very even body-tone and has very good and even sustain up to past fret 26.
The PSG also picked up around 1.5 kilo extra weight because the neck is laminated (screwed, epoxy-ed and heat-baked) of two 10 millimeter layers of "NorthSea hardy" aluminum. I don't really know what "NorthSea hardy" means - it's what the supplier called it, but it is near impossible to polish the surface to a shine - which suits me just fine


As it is: I don't think too much about putting high-tensioned strings on a 3-piece construction consisting of changer-mount, neck (or no neck) and keyhead. That the strings are so high above the top-plate that is supposed to take all the strain means that the two ends of such a 3-piece construction has to be bolted very solidly to a very rigid/strong top-plate to to take up varying string-tension without giving some, and that's were the weakness is found in most PSGs IMO - they are not rigid enough.

The string-tension should be taken up much closer to the strings to lower the leverage and effect of varying tension, which means a 1-piece construction of integrated changer, neck and keyhead doesn't need to be so massive to do the job well (my modified Dekley is over-engineered on this point). With a rigid 1-piece changer, neck and keyhead construction bolted onto a moderately thick top-plate that it can interact with - bolted on in the right places that is, good body-tone, sustain and low body-drop is much easier to tune into a design than what is the case with all these heavy wooden frames that most seem to go for, IMO.

A PSG's frame must be solid - no matter what it is made of, to support the mechanics, legs and top-plate. The top-plate should have some of the characteristics of a sound-board in an acoustic instrument, so it shouldn't be too thick. The string-tension should be taken up above the sound-board - where the strings are, and the mechanical and acoustic coupling between "string carrying frame" and sound-board be limited to a few - well-placed - points for tuning of overall characteristics.


My thoughts above are in line with what I tested out on my Dekley and on a couple of home-built test-objects up through the 1980's and early 90's. Back then I had no real interest in building PSGs, I just wanted one that worked and sounded the way I wanted it to.

Once I had modified my Dekley to my satisfaction in 93, I quit building, testing and modifying and dumped all PSG related projects apart from the Dekley in favor of more interesting and/or important challenges in life. As a result I no longer have any hard data or demo-objects, apart from that Dekley. No big deal to me - I only got one more PSG to design and build, but lack of documentation to back up my thoughts and views may be a problem to others.

Does all that answer your questions Richard?