MID RANGE BOOST
For an overview, see below. For details of the circuit, now see this page. Notes on the distortion, tremolo and reverb circuits to follow.
The mid-range boost effect, which was adopted from the American Super Beatle amps, is perhaps best known on The Beatles' "Birthday" - demonstrated in this amazing video by Andrew Lubman.
Above, a detail from the JMI schematic for the preamps of the solid state guitar amps. The most important components in the circuit are the inductors (L1 and L2) and the switchable capacitors (C14, C15 and C16). In the picture on the right, one can see the three capacitors attached to the switch, and to the right, the inductors.
Above, part of the circuit for the Brilliant Channel in the Super Beatle (designed in 1965) - note the MRB.
In late October / early November 1965, Dick Denney visited America to examine and advise on Thomas Organ amps. The details are given in "The Vox Story", ed. Denney and Peterson, 1993, Appendix 4 (pp. 141-146). Section D, on the Super Beatle, is especially relevant here.
"The engineering prototype of the 100-watt all-transistorized amplifier was examined for its performance and features. ... Tone-control circuits, particularly a resonant-type of boost circuit used in the treble (brilliant) channel, were examined and judged to be satisfactory. The resonant boost feature may represent a new sound in guitar amplifiers and can be carried to any degree of variety by providing multiple switch positions in which different effects can be obtained. Warwick [Warwick Industries, the parent company of Thomas Organ] proposes, at this time to include at least one such selection on a boost switch in the treble channel, and will also reserve provision for, perhaps, a three-position selector switch on the rear control panel which can be used to 'preset' different resonant frequencies."
"The top panel boost switch will be called "resonant boost" and its two positions will alternate between the normal maximum treble boost action, and the selected resonant boost characteristic. An interesting wah-wah effect is obtained by operating the boost switch between its two positions. A remote control for this at the guitar would be novel, however, the cost and complication would be high."