i would have to say that jerry for the most part, as he stated, always used the middle pickup in single coil mode. he may have had a part or song that he would use the others obviously, like the bridge pickup in a portion of throwing stones(dead ahead video). but like his tone controls which were almost a useless function, they were setup for and used when he was using the effects. like giving the phase 100 a humbucker vs. a single coil, or using the extreme low side of the tone control when using the distortion plus. he has stated in almost all i have read on this subject that the key to it was to have the tones seperated from each other. to add different colors. so yes most will find the middle in single coil, or just a single coil in the middle, is ALL jerry. he did things purposely, and those humbuckers and tone control capacitor and resistance values were all selected and used for a reason.
your mileage may vary, but look for those spots in some of those jams and you'll hear a humbucker kick in, or when he rolls the tone control all the way down. it's there, and i totally understand the everyday uselessness of some it, but jerry did use it.
that said, even though there was always the rumor jerry was using a piezo in bolt or tophat, this was untrue. but i do believe he was going to being adding one more thing to his palette of regular guitar/midi guitar setup he last used. there is a bolt schematic floating around that shows just how he was going to employ the new technology of that day and that was some piezo bridge saddles added to that guitar/midi setup. how or if he would have went that route, we will never know, but there is evidence he or his gurus were looking into that setup, and even had custom piezo saddles made and a schematic drawn up.
point is, jerry was always looking for more color he could squeeze into a guitar, and although he did the mjority of the time use the middle single coil, he was all over having as much other colors at his disposal for use when ever he wanted/needed them.
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