Amplifier, Modified, Ex US Military

In December 1948 Radford advertised surplus US military 25 W TG10 Keyer amplifiers modified to work with a microphone or gram and a transformer to work from 110 V to 230/240 V mains.

This amplifier featured:
Circuit Topology

• Output Stage: Push-pull configuration using two 6L6 power pentodes, capable of delivering 25 watts RMS.
• Phase Inverter: A 6N7 dual triode likely served as a split-load phase inverter, enabling balanced drive signals for the output stage.
• Preamp/Driver Stage: Two 6J7 pentodes (possibly RF-type valves) handled initial signal amplification and tone control integration.
• Power Supply: A 5U4G rectifier tube in a choke-input or capacitor-filtered design, originally configured for 110V AC. An optional auto-transformer allowed operation on 210–230V mains. 21/- extra.

Key Features

• Input Flexibility: Modified to accept microphone and gramophone inputs, with adjustable tone controls for broader usability.
• Output Options: Multi-tap output transformer supporting 4 Ω, 8 Ω, and 15 Ω loads, reflecting adaptability for diverse speaker systems.
• Low Distortion: Early emphasis on precision, with distortion levels low enough to necessitate custom measurement tools—a hallmark of Radford’s engineering rigor even in this era.
• Cabinet: Mounted in a smart black crackle carrying case.
• Price: £7/12/ 6 with valves.

These designs laid the foundation for Radford’s later hi-fi innovations, combining pragmatic reuse of wartime hardware with thoughtful circuit optimisation.

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