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Simple Tone Controller 12 Watt Amplifier

Wireless World, February 10, 1933.
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When it is required to provide music for dance halls and clubs, an AC amplifier such as that described by a reader in these notes will be found very satisfactory. The push-pull output stage with provision for careful balancing should ensure high quality reproduction.
Showing the panel with the various controls, including an adjustment for tone.

The amplifier briefly described in this note having proved most successful for public-address use and for providing dance music, a few details might prove of interest to readers of The Wireless World who have a number of components available that could be used or modified for the purpose.

The Circuit Diagram.

With the help of The Wireless World 'Radio Data Charts' several additional filament windings were added to a mains transformer which on test were found to give the correct voltages. For experimenters who take an interest in working out problems of this nature for themselves these charts are a most valuable asset.

The unit embodies two amplifying stages; in the first an MS/PenA valve is employed, and this is coupled by a parallel-fed transformer to two PP5/400 valves working in push-pull. By employing separate filament windings for each output valve, and carefully adjusting the grid-bias resistances, a perfect balance is achieved, so that the maximum efficiency is derived from this stage.

The rising characteristic of the HF pentode used, in conjunction with the tone control - which takes the form of a variable capacitor connected across the secondary of the inter-valve transformer - is very helpful when matching the output to various acoustic surroundings and for correcting gramophone recordings.

The components are mounted on a channel-section aluminium chassis with some of the smaller parts and the whole of the wiring carried underneath.

Since the valves dissipate a considerable amount of heat, a well-ventilated light steel case, having a hinged lid, was constructed to house the amplifier.

An amplifier of this type is capable of giving high-quality reproduction at great volume, and can be especially recommended for public-address;work and the like. The output stage delivers between ten and twelve Watts of undistorted power, which, when fed to a good , moving coil loud speaker, provides ample volume to fill a large dance hall accommodating between three, and four hundred people.

Plan view of the amplifier with lid removed.

Valves: MS/PenA & PP5/400.

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