s.g. brown electromechanical amplifier type v c. 1924

This electromechanical audio amplifier was produced by S.G. Brown Ltd., London in c. 1924. A total of 4 models were available and this is the Type V (for valve sets) 2000 ohms input, 2000 ohms output. A contemporary advertisement for the device is shown below:

Contemporary advertisement for S.G. Brown Amplifiers

At the time this amplifier was produced a 2 valve amplifier would have attracted a 10s 0d BBC royalty whereas the Brown amplifier attracted only half this amount. Also the electromechanical amplifier would have been far more reliable than any valve amplifier of the time and less demanding in terms of battery requirements and usage.

The following picture shows the instrument with its lid opened to reveal the workings and the schematic diagram on the underside of the lid.

 

Operation is as follows:

The input is applied to the  'LE' terminals (top left in the above picture and labelled 'INPUT' in the diagram). The input signal sets up a changing magnetic field in the 'RECEIVING COILS' which cause an iron reed to vibrate in sympathy. These vibrations are applied to a tiny carbon microphone (the brass component shown below, to the left of the large slotted screw). A 6V battery and the primary of a step-up transformer are connected in series with the carbon microphone. As the resistance of the carbon microphone changes in sympathy with the input signal, so the current flowing in the primary of the transformer varies. The external loudspeaker or headphones is connected to the high impedance secondary of the transformer.

 

This particular example is in very good original condition and works.

 

Copyright 2000 - 2002 Lorne Clark