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The VOX sound system

The resultant varying level of brightness, representing the audio on the film was reflected by a small
mirror into the sound photo cell. The amount of this reflected light reaching the PE cell was controlled
by the rotation of a sleeve with a cut out in it around the PE cell. The amplifier did not have any form
of ‘normal’ volume or tone controls. Of course this system had several drawbacks: Any stray light
(from 50 Hertz mains) in the room could cause hum in the sound, even daylight will reduce the sound
level. Most audiophiles will tell you that running the amplifier ‘flat out’ and controlling the sound in
such a way creates the worst possible ‘signal to noise’ ratio.
The same principle of operation of the sound system was also used the Pathé’s 17.5mm Home Talkie
projector of a similar vintage.
The rotary stabilizer is not particularly efficient, there being only a single spring loaded arm holding
the film against the flywheel assembly. In operation there is some ‘wow’ as the film varies a little in
speed as it crossed the sound pick up point. (Many of the sound tracks on early Pathé films were
recorded with varying degrees of ‘wow’ in the sound, so I guess one should not be too critical).The
amplifier consisted of a 6U7 preamplifier, a 6F5 phase splitter and a pair of 6F6 in push-pull as
outputs and a 5V4 rectifier. The original valves (tubes) fitted were made in the USA and were the
smaller metal envelope types (as opposed to glass), which were very common in American made
equipment during the war years.
Speed change is via a knurled knob on the off-side of the machine, which engages one of two
different diameter drive gears.
The film path is off-set, with to feed sprocket (top) being further towards the centre of the body to
allow the complete lens system to lift up and hinge forward for easy threading and cleaning. The front
acting claw system housing is located under the lens.
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