Recording of 'Unplugged'
The acoustic guitar Kurt used on "Unplugged" ("Unplugged 21")(and his main acoustic guitar around this time) was his late 50's Martin D-18E (57, etc).
Although the standard procedure for "Unplugged" is for acoustics to run direct, Kurt insisted on putting his Martin through his Fender Twin Reverb amp; but it was only used as a monitor! To keep the amp as clean as possible, the 7025 preamp tubes (59) were replaced with 12AX7's and the 12AT7 phase inverter was replaced with a 12AU7. Alex Coletti ("Unplugged" producer) made a box out in front of the amp to make it look like a monitor (32)(57). Again, Earnie believes this amp was used ONLY as a monitor, as the guitar on the album sounds to him as if it's coming off of the D.I. box. He can't be sure, though (59).
Along with the Twin Reverb, the DS-2 distortion pedal and E-H Small Clone chorus were also used (seen 21)(57). The Echoflanger and Polychorus were also brought to the rehearsal but they weren't used because they created too much 60-cycle hum (57).
Kurt used Finger-ease on the fretboard of the Martin to smooth out the solo to "About A Girl." "You know that goofy-ass stuff? It's like anal gel" said Kurt, who knew to use the roll-on as opposed to the spray. He said that he had never used fretboard lubricant before but that his "country and western aunt" used to (32).
http://www.kurtsequipment.com/recordingpage.htm#unplugged
7). Martin D-18E acoustic, also see "Recording of 'Unplugged'" section. Late 1950's, serial # 166854 (59), bought at Voltage Guitar in Los Angeles in the fall of 1993. The D-18E was one of Martin's earliest stab at electric guitars. Only 302 were made before it was discontinued in 1959. It is basically a D-18 acoustic but with two pickups, three control knobs (one volume and two tone), and a selector switch. Kurt probably didn't have any idea how rare the guitar was when he bought it, according to Earnie Bailey. Voltage recut the existing nut so a left-handed Kurt could play it, although out of intonation, probably due to Kurt "needing it yesterday." "I cut the new nut, and John Saba (longtime friend and Seattle area repairman who specializes in tricky acoustic bridge work and neck resets) filled in the bridge and routed the new slot for the bone piece. Kurt liked the work and sent me the Epiphone (Nixon Now) for the same treatment, but John replaced the entire bridge for that I believe. John now works for Dusty Strings, a high end acoustic & bluegrass shop in Seattle"(57).
The D-18E didn't sound very good, so in addition to the instrument's DeArmond pickups (which were designed to be used in conjunction with nickel strings and sounded poor with bronze-wound), a Bartolini 3AV pickup was added to the top. Kurt was interested in the pickup because of its use by Peter Buck (21)(29)(32)(57).
http://www.kurtsequipment.com/guitarspage.htm#acoustic
