The following diagram shows the classic Marshall / Fender tone control circuit. In a Marshall or Fender amplifier, this is usually installed between the 2nd and 3rd 12AX7 amplification stage. You might choose to place this somewhere in the "multi-variant" pre-volume arrangement, or just before the phase splitter stage. Experiment! Probably the only completely inappropriate place to install this circuit is just after the input jack, because the gain there is so high that you'd never be able to wire it in a way that wouldn't pick up a lot of noise. Note that the Bass control is wired counter-clockwise from the schematic.
|------> to next pre-volume control (1 meg)
| _________ ________
| | | | |
From Plate >-------/\/\/\------/\/\/\-------/\/\/\-----------> (ground)
of 12AX7 | R1 | R2 | R3
--- Treble --- Bass --- Midrange
Fender: --- --- --- Marshall:
| C1 | C2 | C3
R1 = 250K ----/\/\/\----------------- R1 = 250K
R2 = 250K R4 R2 = 1M, log
R3 = 10K R3 = 25K
R4 = 100K R4 = 56K
C1 = 250 pf C1 = 250 Pf
C2 = 0.1 uf C2 = 0.022 uf
C3 = 0.047 uf C3 = 0.022 uf
To install the post-volume control, there's basically two choices for its location in the circuit topology. One is just before the phase splitter tube, and this requires only a single potentiometer metering the level into the grid input of that stage. An arguably better choice is to place this control just before the two output tubes. However, you'd need a dual potentiometer in that case, in order to meter the voltage to both tubes. This arrangement is considered better because it includes the sound of the phase splitter tube. There's no reason you couldn't do both, ending up with a multi-variant arrangement of two pre-volumes. In an Integrated amplifier, you'd only be limited by the number of control locations on the front panel.
