In general, it depends on what kind of music you are working on. There is plenty of organ music which was written for manuals only. Obviously, to play such music on the Spinet organ is no problem at all. In addition, a significant part of early organ repertoire was composed with a short pedal board in mind.
For example, Italian organs for many centuries didn't have a full pedal board so anything Italian would work fine on a Spinet organ. The question remains what to do with the classical organ music, like the music of Bach which often requires 27 note pedal board (sometimes even 30)
In general, for music which requires the full compass of pedal board you have only 2 options:
1) To arrange the pedal part so that it will fit the short compass of the Spinet. For example, notes in the pedal part above tenor c would have to be played one octave lower. Sometimes an entire excerpt might be played one octave lower.
If you have to play notes from c sharp up to f in the treble octave, you can lower them by two octaves. In doing so, you may also have to adjust the pedaling. For example, this could mean that using the right foot on the Spinet organ might be complicated so the majority of notes should be played by the left foot.
2) To play as written, imagine the additional pedals, and press the approximate spot on the floor. It is also possible to add a wooden board on the floor of approximate the same height as the Spinet pedals so that you will have the same feeling while playing with your feet. In addition, you can draw the missing pedals on this board so that you will know exactly where to play.
If you want to play classical organ music on the Spinet electronic organ, use the above tips for pedal playing. It is also a good idea from time to time to get access to the real pipe organ. Occasional practice on a full length pedal board will allow you to have the correct feeling for your feet.
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