The pickup and circuitry installed in the single pickup Gibson EB4L bass guitar was quite distinct from other EB basses. The EB-4L pickup too, although externally quite similar, was of a very different design to the Humbucker fitted to the EB-0, EB-2 and EB-3.
According to the original EB-4L schematic (see below) the loom itself consisted just one 300kΩ volume and one 250kΩ tone control, a three-way detent switch (part 70782), and a choke (part 70442 - the same as used in the Ripper). Capacitors are 0.1µF and 0.47µF. The switch is quite unique, and is not used on any other Gibson instrument* - nor is it the same type fitted to typical Fender guitars. In fact it was a fairly new invention at the time. It carries patent number (3,261,931, and is entitled "Lever Switch Detent Apparatus"). *Actually a similar switch is fitted to the Les Paul Triumph bass, though it is listed with a different part number, 70481, in Gibson literature. Whether there was any crossover between these parts is currently unclear.
Despite the suggested pot values, often two identical 500kΩ pots were used (typically part 70027).
The volume pot in this early '73 EB4L circuit is clearly marked part 70027, i.e. 500kΩ. The tone pot codes are covered in solder and illegible.
The patent number on the switch casing is clearly visible. Patent number 3,261,931.
Two versions of this switch are shown above - despite dissimilar appearance, both have the same patent number and were bought as new old stock from the closed down Kalamazoo plant. The page in the background shows the switch in the 1977 Gibson spare parts catalogue (U Lever switch, part 70782). The type fitted to the 1973 bass above (in fact all 1973 EB4Ls examined to date) more closely resemble the switch on the left. The 1977 illustration resembles (but is certainly not identical to) the switch on the right. With the bulk of EB4L production occuring in 1973, it would seem that the left-hand switch is the norm, with potential use of different, but equivalent parts later in the decade.
Does your EB-4L use a different switch or different potentiometers? comment
The original schematic for the EB4L bass does not specify part numbers for the tone or volume pots, but does offer a number for the tone choke.
Gibson EB-4L bass guitars for sale
GIBSON EB-4L BASS - 1972 with Original Case
Redditch, Worcestershire, B98***, UNITED KINGDOM
£1499
