Ned McIntosh
From:
New South Wales, Australia
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Posted 15 Aug 2010 11:31 pm |
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Simple answer:-
Push-pull changers use a split changer-finger, actuated by rods driven by bellcranks to raise and lower. Raising requires the raise-rod to pull the raising-section only of the split-finger to effect the raise. A lower requires both the raising-section and the lowering-section to move simultaneously backwards to reduce string tension so the lower can occur. Lowering also requires the "lowering-rod" to push against the changer-finger, instead of pulling on it.
One "feature" of the push-pull system is that raises always win out over lowers if both are on the same string and are engaged simultaneously (string 6 on E9th, commonly). This may be regarded as a feature by some, or a "mis-feature" by others.
Due to the requirements to pull a rod (for a raise) and push a rod (for a lower) the undercarriage on a push-pull can be quite complex and intimidating to the novice. In particular, if a string is both raised and lowered, then slack must (repeat must!) be present in the raise system to allow the lower to take place. The two brands using push-pull are older Emmons and Pro-Mat.
BTW The older "pull-release" system is basically the same but has a very simple single-piece changer-finger (Marlen used this on their earlier steels until they went to an all-pull changer.)It has one rod for raising and one rod for lowering, which is fine for simple copedents but mitigates against complexity. Properly set up it is a very reliable system. Again, a string that is raised and lowered must have slack in the raise system for the lower to work properly.
The All-Pull changer is a very different beast indeed! It uses a more complex changer-finger with a "knuckle" in it, around which the upper and lower sections may pivot. Pulling on the upper section of the knuckle causes a string to raise, pulling on the lower section of the knuckle causes the string to lower. There are a couple of variations as to the way this is achieved, but basically the rod is pulled, no matter whether raising or lowering, hence the name.
A feature of all-pull is the ability to balance the "in-between" note on a string that is both raised and lowered, giving the ability to have what is commonly referred to as a "half-stop". Commonly this is used on string 6 on the E9th neck, which is raised to from G# to A on a raise and lowered from G# to F# on the lower. The ability to accurately tune the G note when both the raise and lower are engaged at the same time is useful for getting a minor chord at the root position on strings 4,5 and 6.
All-pull changers typically have a set of return-springs, leading to a more complex design than push-pull or pull-release. However, it is typically much, much easier to alter the copedent on an all-pull changer than on a push-pull. Complex copedents are now almost the exclusive domain of the all-pull system.
The downside of an all-pull changer is this:- for every lower (via a pedal or a lever) on any given string, a separate lower-rod is required, ditto for raises. Thus we have the development of triple-raise, triple-lower changers, meaning each changer-finger can have three separate raises on it and three separate lowers. (It does not mean the changer can raise three half-tones and lower three half-tones!) The undercarriage of a complex copedent on an all-pull can get very crowded (see the photos of Greg Cutshaw's MSA on his website for a good example). Mind you, a fully-loaded Emmons push-pull isn't exactly a walk in the park underneath, either!
That's it in a nutshell. I won't talk about tone of one versus the other because I have four steels, three of which are all-pull, one is pull-release. They all sound great! However, it is believed by very eminent and learned players of many decades experience that the push-pull system gives a tone which is unobtainable from all-pull. Notice I said "unobtainable" and not "better" or "different". I cannot say the tone of one system is better than the other because tone is extremely subjective!
The argument about tone will likely fill all the available bandwidth of the Internet, given time. All I know is both systems work, and work well, when properly adjusted. Correct adjustment is critically important!
I am sure forumites who are far more knowledgable than I will clarify my explanation, which I for one would welcome. On the Carter website (still working as of today) there is a very good animation of how a push-pull changer raises and lowers, as well as two slightly different types of all-pull changers. It is accessed via the "PSG illustrated" link, parts function map top view, select the changer on the photo and follow the links for the animations. Very informative. (Bob Hofnar's links will take you straight there!)
I did try to keep it simple, honestly I did!  Thanks also to Donny Hinson who helped me tighten up on some slightly wayward terminology, much appreciated.
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The steel guitar is a hard mistress. She will obsess you, bemuse and bewitch you. She will dash your hopes on what seems to be whim, only to tease you into renewing the relationship once more so she can do it to you all over again...and yet, if you somehow manage to touch her in that certain magic way, she will yield up a sound which has so much soul, raw emotion and heartfelt depth to it that she will pierce you to the very core of your being. |
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