RF power amplifier

Home built in the UK in 1956.

Like much equipment built by radio amateurs for experimental work, this amplifier has undergone several mutations during its lifetime. It started out in 1956 as a 14/21/28 MHz power amplifier using a pair of countersunk 5B/254M ('miniature 807') valves in push-pull class C. Later an electronic T/R switch and receiver preamplifier were incorporated, and during the sunspot minimum years the 28 MHz range was removed and replaced by larger input and output inductors and a suitable RFC for 3.5 MHz operation. In its last configuration, shown in the photos, it used a pair of TT21 valves with anode suppressors instead of the 5B/254Ms, and the amplifier was still used until the mid 1960s. 

Input from the VFO/exciter is by coax to link coils on the switched tuned input circuits, while the output is a Pi-network using Eddystone 836/839 transmitting variable capacitors. 20 mA grid current and 250 mA anode current meters are provided. The PA was used mainly for CW, NBFM and RTTY, but an AF choke is provided in the screen grid supply so that AM was possible by anode modulation alone. (The modulator is the item at bottom right in the shack photo. It used a pair of 807s in Class B zero bias). When switched to CW operation the choke is shorted out and the grid bias, which is provided by metal rectifiers, is reduced automatically, 

Valve complement:
2 TT21 Push-pull PA.
6C4 Electronic T/R switch and preamp.
Metal rectifiers.

Power supplies: 220 - 240v AC and 700v DC.

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