“Considering the fact that Frank rarely used anything standard, it’s hard to imitate his sound. The important thing is to have the right mid tones and to have the feedback controllable at will.
Frank devised the system to do this by equipping his guitars with preamps that put the EQ control at his fingertips and fired out up to 18dB of boost. Frank loved to tinker with this stuff and managed to make it work for him and his playing” (Dweezil Zappa).
This then is a selection of the guitars that Frank used most often, or that he particularly loved.
Gibson ES-5 Switchmaster
This Gibson ES-5 Switchmaster was played on the first 3 Mothers of Invention records. It was also played on the album “Freak out!” amplified with a Fender Deluxe. Note the stout equipment of potentiometers and switches and the Barcus Berry pickups, implemented later.
SG “Baby Snakes”
The SG “Baby Snakes” was Frank ‘s most frequently used guitar in the latter part of the 1970s. It was not
actually a Gibson, as it looks from the shape, but instead a creation of “a Phoenix guy” who managed to get backstage and sell the guitar to Frank for $500.
In fact, while very similar to the official model, it had a 23rd fret and some embedded wooden ornaments and details. Luthier Rex Bogue also added other goodies to this guitar, such as phase switches and a preamplifier built into the guitar body.
Gibson Les Paul Custom
This is the guitar Frank is holding on the cover of “Shut up and play yer guitar“. It is a Les Paul Custom equipped with 2 Seymour Duncan humbucking pickups, a Dan Armstrong Green Ringer electric circuit and an XLR output.
An extra potentiometer, positioned between the original ones of the house, is in effect a 9-position switch that allows you to choose between single coil\humbucker and out-of-phase options. The toggle switch chooses whether to operate in series or parallel.
SG “Roxy”
This is the guitar used in the 1974 album “Roxy and elsewhere,” but it was later implemented, in addition to the 2 switches on the lower horn of the guitar, already present in 1974, with the black pickguard and various preamplifiers, phase switches, and active filter circuits.
Martin D-18S 12-fret “Standard” Dreadnought
This is a Martin D-18S “Dreadnought” with a slotted headstock, Brazilian rosewood fingerboard and bridge, and 
mahogany body. It was used in the recording , in 1974, of “Sleep dirt” and in the song “Blessed relief” from the album “The Grand wazoo.”
Frank traded it for one of his own Telecaster with its previous owner, Mark Volman, i.e., the “Flo” of the Mothers of Invention line-up of the early 1970s, up to the album “Just another band from L.A.”
Performance Guitar
Played by Frank on his last tour in ’88, this custom solid-body performance guitar sports concentric potentiometers together with screwdriver-operated trimpots to adjust the squeaky timbre Frank was characterized by.
Midget Sloatman, one of Frank’s engineers, stated in 1995 that the trimpots were identical parametric filters, one for low frequencies (50Hz – 2kHz) and the other for high frequencies (500Hz – 20kHz).
Filters
The filters also had a potentiometer to act on the resonance (Q) that allowed Frank to control the feedback characteristics of his instrumentation in any concert environment. Frank basically tested his guitar on how the hall responded to the amp, and then used the Q pot to get the feedback he wanted.
He also used ai filters on the high frequencies (4k – 8k) to bring out some nuances of sound from the strings.
Jimi Hendrix Sunburst Fender Stratocaster
This guitar was given to Frank by one of Jimi Hendrix’s stage assistants. It was the Fender Sunburst Stratocaster that Jimi mutilated at the 1968 Miami Pop Festival.
After having kept it hanging on the wall of his studio for years, he entrusted it to Rex Bogue (the same luthier\electronic technician who implemented the Baby Snakes) who brought it back to life by also mounting all the electronic devices that Frank was crazy about, such as the Dan Armstrong Green Ringer or the Barcus Berry embedded in the handle that functioned as a kind of piezo.
The original pickguard and maple neck have now been replaced.
Guitar Player
This guitar has also been played live by Steve Vai and other guitarists in Frank’s band, as well as appearing on the cover of the renowned American magazine “Guitar player” in 1977.
No doubt this is not an exhaustive list, it would be difficult to do so for an artist with over 70 official records released, but a personal selection of the most significant instruments of his career.
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