This photo shows the final amplification stage of the Marconi S.W.B.8W HF radio transmitter. The final valves (left and right), with aluminium radiators above, generated the final power amplification which was fed into a tuned circuit (behind and at top). The black knob at centre bottom controlled the tuning condensor (capacitor) - the horizontal plates between the two valves - which formed part of the tuned circuit.

High power output (up to 3.5 KW) was required to overcome atmospheric noise at the lower frequencies used. The power amplification process was quite inefficient, with a great deal of waste energy produced in the form of heat.

(Photo: CAHS collection)

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