How do I assemble a cable to make the relay switching work for my Mesa Boogie amp ?
Quad Preamp ?
For the Nomad: Please have a look at this diagram: NOMAD_GMajor.pdf
For the Rect-o-verb (version 1): Please have a look at this diagram: Rect-o-verb_GMajor.pdf
For the Quad Preamp: The G-Major can switch between two channels (which according to the Quad owners manual can be blend if both channels are on simultanously): Use a Y-split cable (TRS stereo into two mono jacks) and connect to (the two) the channels switch inputs you want. If you need full 4 channel switching (and perhabs EQ and Reverb switching) you will have to go for a MIDI Matrix device (MesaBoogie once had such a MidiMatrix, but we cannot find any info on their site)
Other units are available e.g. from Midi Solutions (Relay or R8).