1967 - 1969

Some notes on the mid-range boost circuit, which initially had three positions (as in the American Thomas Organ amps) - - and later six positions after a redesign by Vox Sound Equipment Ltd. c. 1969.

The six-position boost can be seen in , for instance.


A detail of the JMI mid-range boost circuit.

A rough sketch of the circuit as it is physically arranged in JMI amplifiers.

Everything but the inductor and the MRB select switch sits underneath the top of the control panel. Lines in purple show the circuit without mid-range boost. Orange/brown shows how the MRB feeds in.

The mid-range boost selector - "MRB Effects" - has three positions, and only works when the switch on top of the control panel is "ON".

The signal feeds into the treble pot from the treble/distortion circuit board, from there it goes both into the bass network and on to the lower leg of the 1uf capacitor. It then passes across the inductor, into the 10K resistor, the 0.033uf capacitor and finally to the volume pot.

Pressing the MRB switch puts the inductor in parallel with the MRB capacitors, activating the boost. The signal is mixed with the "clean" signal through the 0.01uf (10,000pf) capacitor on the tagstrip.


Above a detail from OS184 - the undated schematic of the treble preamp prepared by Vox Sound Ltd, probably c. 1970. The six position mid-range boost is already found in amps produced by VSEL in 1969, however.

In the redesigned circuit, the MRB has six effects: three when the switch is "OFF" and three when "ON".

A rough drawing (again) of the circuit when the switch on the top panel is "OFF".

The circuit when the switch on the top panel is "ON".

SWITCH OFF. The signal enters the treble pot and passes out to the MRB switch (terminal 1) via a 0.047uf capacitor. It also passes to the bass network (which has a 1uf capacitor across it on the tagstrip). From the bass pot it travels through the 100R resistor on the tagstrip to terminal 3 on the MRB switch. Terminal 3 is connected by a short wire to terminal 2.

The signal passes from terminal 2 on the switch to the tagstrip, through the 0.1uf resistor, and on to the volume pot. When the switch is off, terminals 3 and 4 are connected, so the inductor and three MRB capacitors are active over the circuit.

SWITCH ON. As above, the signal is routed through the tone pots to terminal 2 on the MRB switch. It is also routed to terminal 6.

When the switch is on, terminals 2 and 5 are connected, as are teminals 4 and 6. The MRB effect is therefore applied over the signal via the 100K resistor on the tagstrip.

The mixed signal passes from terminal 2 on the switch out to the tagstrip, through the 0.1uf capacitor, and on to the volume pot.