Transmitter AD57/A

The Marconi AD57 was a dual‑band airborne and marine transmitter developed in the mid‑1930s for Imperial Airways aircraft (notably the Armstrong‑Whitworth Ensign) & maritime communication. It handled CW, ICW, and AM telephony modes, paired with the AD5872 receiver/DF system for long‑range operational and navigational communication. The output was roughly 65 W, suitable for long-distance HF contact.

Construction and Power Supply

• Enclosed in a cast aluminium alloy case on anti‑vibration mounts for aircraft/ship service.
• Operated from a 24‑V DC aircraft supply through a motor‑generator (~3,500 rpm) providing 1,200 V HT and 12 V LT outputs.
• Contained two complete transmitters: one for medium-/long-wave (MW), one for short-wave (SW).
• Front panels & valve bays were easily removable for servicing.

Medium‑Wave Transmitter (500–273 kHz)
Modes: CW, ICW, AM
Power output: ~66 W
Range:

• CW: 400 mi
• ICW: 300 mi
• R/T voice: 200 mi

Main valves and functions:

• Master oscillator: Marconi DET.6 triode – generated the carrier frequency.
• Power amplifier: Marconi ACT.6 triode – boosted RF output to aerial power level.
• Modulator: Marconi MHL.4 triode – provided grid‑leak amplitude modulation for voice; up to 60 % depth.

Keying and modulation:

• CW/ICW used negative HT line keying for clean signal shaping.
• ICW modulation derived from a 1 kHz interrupter disc on the generator shaft.
• AM modulation applied via the MHL.4 to the oscillator stage.

Operator controls:
Tuning condenser (with five pre‑set spots), CW‑ICW‑Telephony selector, aerial tuning & variometer controls, and neutralizing (“Neutrodyne”) adjustments.

Short‑Wave Transmitter (17.76–4.00 MHz)
Modes: CW only
Power output: ~60 W
Range: 500–3,000 mi (depending on HF propagation)

Main valves and functions:

• Master oscillator: Marconi DET.5 triode – frequency source.
• Power amplifier: Marconi ACT.6 triode – RF output stage, similar to MW side.

Control features:
Five click‑stop oscillator presets, aerial tuning controls, wave‑range selector switches, and Neutrodyne neutralising. The same HT keying method was used for CW.

Performance Summary

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Tous les appareils de Marconi Co. (Marconiphone, Marconi's Wireless Telegraph Co. Ltd.), GB (not tubes - see also HMV His Master's Voice)