<Multidimensional Oscillator >
an experimental complex sound generator
In 1971 we designed and made our 'Dodekadent' for the Logos group as an instrument for live-electronic music performances. That instrument used twelve relaxation oscillators using neon-tubes. These make simple and cheap sawtooth oscillators. An inherent property of such oscillators is that they are voltage controllable and thus very suitable for using in automation and sequencing circuitry. During the construction of that instrument we noticed that the small neon-bulbs we used, when exposed to a modulated light source, generated much more complex waveforms, including frequency modulation, than the regular saw-tooth. This gave us the idea to experiment with sound generators taking profit of this effect. As we had a few gas filled display tubes available (they were actually given to us by technicians working at the computing labs based at Ghent university) with many electrodes, we designed a series of complex sound generators using a single decatron tube with two or three mutually interacting oscillating systems. Here is the pinning of the base socket:
We used this sound source in the early years of Logos but due to the inherent unpredictability of the sound results, abandoned it after some time. When Mattias Parent started cataloguing many of our old electronic instrument designs in 1998-2020, this device came into our mind again. I couldn't find it back in the storage of the labs at logos. However, I found back many of the original components: the rotating capacitors, the decatron tube and parts of the chassis, the partly disassembled high voltage power supply. Missing parts were the original 10MOhm potentiometers, the same type also used for the construction of the Dodekadent, but nowadays absolutely untraceable on the component market. As we found the decatron tube to be in good working condition, rebuilding became a possibility. In 2020 we decided to try to reconstruct the original design, partly from left over documentation, partly from our own memory.
Here is the reconstructed circuit drawing: The resistors drawn and labeled as 6M8, were in the original design 10 M Ohm potentiometers. An alternative would be a circuit like this, using a 12-position rotary switch with fixed resistors: At the time we were dreaming of using all cathodes with an individual relaxation circuit, but fast enough we found such a circuit to become completely unmanageable. Even with three relaxation circuits, it was very difficult to start it up properly. So, the practical circuit used only two oscillators, as drawn here. However, the steering electrodes can be used reliably for further modulation. For coupling, audio transformers with high impedance windings should be used. Such transformers were recycled from old audio vacuum tube equiment (mixer boards and professional tape recorders).
A webpage on the decatron GS10D tubes used in this experimental instrument.
Valve socket: B12E CTR Holder, cat. nr. 34/1201, Ediswan Clix radio components.
Last update: 2020-09-29