CASTLEDINE ELECTRONICS: Magical Mystery Box

Since I've been on a slight Beatles related run lately, I thought I should tackle the Castledine Electronics Magical Mystery Box.  

This is Stu Casteldine's version of the Brilliant channel from a Vox Conqueror guitar amp.  Castledine included a gain recovery section at the end and the transistor-based distortion switching has been removed.  I ran both the Castledine version and the original in LTspice, and there was no real difference between the two.  

The thing that really sets these apart from similar Vox amps is the mid-range boost (MRB), which is said to be the precursor to the wah pedal.  The MRB has three different options for the mid-range boost frequency - the sound is what we now refer to as a cocked wah.  i.e. turning your wah pedal on a leaving it at a specific frequency.

As this line of Vox amps were transistor-based, the pedal version will be much closer to the sound of the original than a lot of other amp-to-pedal conversions that just replace valves with JFETs.

CASTLEDINE MAGICAL MYSTERY BOX VERO LAYOUT  

I've not done a layout with a Fasel inductor before, so hopefully I have the sizing about right.   The layout is intentionally wide, as this will be in a large box in landscape orientation (three foot switches and five controls along the top).  

Despite the size, you may need to add some space to mount this using your preferred method.  I strongly suggest testing the layout and sizing before starting construction.  The fasel inductor sits quite high compared to other components.  It would be annoying to get to the end of a more expensive than normal project, only to find that it doesn't fit in the box.

VOX CONQUEROR / DEFIANT / SUPREME PRE-AMP

Several Vox transistor amps shared the same pre-amp including the 30W Conqueror, 50W Defiant, and 100W Supreme.  They were experimenting with modular amp designs at the time - probably to save money and streamline production.

The Brilliant channel is the section on the top-left to centre.  If you compare this to the Magical Mystery Box, you can see they are very similar. 

When the distortion was switched on, it provided a bias current to the base of the transistors, allowing sections of the distortion circuit to connect to ground.

This is a redraw of the original schematic by Chris Devine of Back to the Sixties (link below).

CASTLEDINE MAGICAL MYSTERY BOX

ADDITIONAL VOX RESOURCES

Vox Showroom - https://www.voxshowroom.com/uk/amp/conq.html

Back from the Sixties (scroll down for schematics) - https://www.backfromthesixties.co.uk/vox-supreme