Electronic organs - circuits, experiences and interests

[Lucien Nunes]

The idea was basically a one octave LF keyboard, a bit like a sub Fender Rhodes bass keyboard but lower frequency, eg 16.35hz & up. I tried a phase shift oscillator but that wasn't a success, it was too hard to adjust & the sine wave was distorted. I couldn't find a simple valve oscillator so experimented with a op-amp wein bridge oscillator adjusted using 10 turn presets. With that I could dial in the frequency so it was bang on.

Your experiments remind me strongly of those by Leslie Bourn, inventor of the Compton electrostatic tone generator. If my memory serves correctly, he told a story to the Electronic Organ Constructors' Society in the 1960s, of his efforts to make an organ bass pedalboard on which to practice at home, to save going to the cold, empty church in winter. Having been demobbed from Signals Dept. in WW1, he had scrounged some bright-emitter valves, but by the time he tried to use them the filaments were found to be dud. He also tried tuned mechanical vibrators (buzzers), I can't remember the outcome. But in describing these labours to Kenyon Hope-Jones, brother of Robert the organ-electrification-wizard, he triggered an introduction to John Compton, who invited him to join the firm with the remit of developing electrical tone generation for organs. He tried electromagnetic generators with audio keying (like Hammond), then with DC field magnet keying, but eventually settled on DC polarisation keying of electrostatic generators which debuted in 1935, the same year as Hammond.

If you want to experiment with valve oscillators you will probably do best with a Hartley circuit but you will need to wind or adapt some coils. A tank at least, possibly a separate tuning coil with adjustable limb. Here are some pics of a commercial implementation, from an early 1960s Miller. The tank (A) is tapped to allow selection of take-off amplitude. The tuning coil is (L). An anode coil with variable coupling (B,C) is included to collect a harmonic-rich waveform. For a small number of oscillators you could do this more easily with RC coupling, but in this organ there are hundreds of these coils wired in series where RC would be too lossy. Some filtering might be wanted downstream to form the best tones but the output of the tank coil is quite pleasing in itself.