Photographs courtesy of Derek Batten
You are now looking at a Series Two Melotone unit.
The mode of tone generation is electrostatic, as distinct from the Hammond electromagnetic principle. Each note is produced by an undulating electrical potential, generated by the relative rotation of two parts – one of which carries a wave or undulating form, and the other, a scanning array for scanning said waveform. A Melotone generator assembly consists of two stator discs – one either side of a double-sided rotating scanning disc. There are two octaves of undulating waveform rings on each of the stators – natural notes of the scale are on one, and sharps on the other. In order to cover the six-octave compass of the Melotone, two such generator assemblies are employed: a bass disc, generating the two bottom octaves from Tenor C (utilising two of the four available octaves), and a treble disc generating a further four octaves. They appear respectively left and right in the photograph.
Bass and treble discs are identical, but speed of rotation - and consequently, pitch - is governed by the size of drive pulley (23). The treble disc revolves at four times the speed of the bass disc. The close-up photograph clearly shows the larger drive pulley for the bass disc, and a proportionally smaller diameter for the treble.
In his patent applications, Leslie Bourn specifies a graphite coating, but in the actual production, a more satisfactory coating of nickel appears to have been used.
This coating is then divided into concentric rings by means of longitudinal sine wave cuts (a) of the required wavelength. The simplified diagram below represents a segment of a stator disc, but with the waveforms on a straight path. It will be noted that there are two engravings (a) per note - two sine waves in antiphase - the innermost (and consequently smaller-diameter) waveform being delayed by half its period. The engraving process effectively leaves the waveform rings (1) isolated from each other, and the active surfaces of the stators - including both the waveforms and the sections between - are electrically connected to wire pins, passed through the moulding. The waveforms (1) are then connected to the time-constant circuits and the sections (4) between linked together and rendered ineffective by being earthed. It will be noted that the highest note is to be found at the outer edge of the disc, and the lowest at the centre.
The concentric rings (2a) and radial scanning members (2b) of the grid electrode (2) are formed of graphite (or other conductive medium), located in the channels engraved in the insulating material of the rotor disc (5). After engraving, the disc is completely coated with a graphite or conductive layer, and the surface then rubbed down, to leave only the infill in the channels.
The common scanning electrode takes the form of a grid (2), consisting of a number of concentric ring supports (2a), which are superimposed over the respective spaces between the undulating rings of the stator disc. Short radial scanning members (2b) extend between each adjacent pair of these concentric rings, the radial members of each set lying transversely over a respective undulating ring (1). These scanning members are spaced by a distance equal to the wave length of the ring being scanned.
Design of the original Series One/Two discs proved to be a disaster. An acetate-based form of plastic appears to have been used, which created the problem of it having to be kept at a relatively stable temperature. This was all the more critical as the acetate layer was riveted onto an aluminium backing (flywheel) disc. Ian Macnaught (Scottish Cinema Organ Trust) sums up the situation thus: “If the disc temperature was allowed to go up and down – like hot during the day, and cool at night – it kneaded the plastic, causing a catalytic action, which resulted in a vinegar-smelling gas to be given off. The loss of mass in the material then allowed decay of the acetate, into a ‘toffee’ consistency, which gradually poured itself out of the bottom of the disc assembly. The discs became warped, corrugated, and because of the tight limit [between rotor and stator], allowed them to scuff, eventually sticking firm. Normally, the Melotone cabinets were kept stable by the chamber heaters, or by a small bulb in the base of the cabinets”. The accompanying picture – also courtesy of Ian – dramatically illustrates this point, the casualty in this instance being a disc from the Mayfair, Aigburth Melotone.
As notes are keyed, a DC voltage – around 400V at source – is passed to the appropriate rings on the stator plate. Each key contact is connected to its proper generator ring through an adjustable resistance network. The resistors, resembling cartridge fuses in construction, are contained in a series of tray assemblies (three resistors per note), enabling values to be substituted, in order to regulate each note. A capacitor (18) is also connected between each generator ring and earth. The purpose of this is to cause the potential of each ring to build up slowly, thereby avoiding a sharp attack.
From this source of potential, an undulating potential is induced on the scanning element (2), of a frequency depending on the speed of rotation and the distance apart of the undulations of the ring. This undulating potential is then collected from the drive shaft (22) via a carbon brush, and passed to a common amplifer and speaker.
Vibrato is produced by an eccentric drive, which effectively rocks the two stator plates back and forth. This mechanism is clearly visible in the front centre of the generator assembly photograph, and is driven from the main motor shaft by a belt, connecting with the large split pulley on the vibrato shaft. One side of this pulley is coupled to the shaft, while the second side is an idler. When Vibrato is not selected at the console, the drive belt runs by default on the idler side of the pulley. Depressing the Vibrato tab, operates a relay arm, bearing a forked channel, through which the vibrato belt passes. The travel of this fork piece is sufficient to guide the belt onto the fixed side of the pulley, thus activating the vibrato drive.