These are the schematics and layout I used to create my 50 watt guitar
  amplifier.  It has a pre-amp that can be configured as a Marshall 1959, 2204
  or SIR #39 by either a panel mounted rotary switch or a two button foot
  switch.

  The 1959 configuration does have two channels (like the original).  The
  brite and dark channels have seperate volume controls and are internally
  jumpered togeather.

  The amplifier also has two master volume controls.  One is a pre-phase
  inverter master (like the 2204) and the second is a post phase inverter
  master volume (LarMar MV).

  One button of the footswitch controls the relay that selects between the 1959
  configuration and a "boost" configuration.  The boost configuration is either
  a 2204 circuit (one gain stage more than a plexi) or the SIR #39 (two gain
  stages more than a 1959).

  The schematics (.sch) were done with kicad.

  The layout was drawn with librecad (.dxf) and is to scale 1 = 1 inch.  So,
  the layout can be used as a template for drilling the circuit board and
  chassis (which is exactly what I did).