From the book of sounds by David Steven. Click on pic to enlarge!
a. Follow normal two-osc. tuning procedure. Osc. 1 is an octave above osc. 2.
b. Start with output channel 2 level set at 0 (output channel 2 not monitored).
c. Play middle C and advance output Channel 2 level until note breaks up (feedback). Back control off slightly.
d. Play slow glissando from bottom to top of the keyboard tp check that all notes play normally. If any break up or are octave-doubled (i.e. note and its octave), retard the output channel 2 level while holding that note until the note is stable.
This feedback technique adds a considerable number of timbral possibilities to the Synthi A. Experiment with this patch by altering osc. levels and waveform shapes, and filter frequency controls, (filt/osc. 2 output filter). Also increase output channel 2 level by varying amounts and see what happens.