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Vintage Radio

British manufacturers built some superb high-end audio amplifiers during the 1950s and 1960s and the Leak TL/12 was one of these. It's a 5-valve mono amplifier with some interesting design features and is reasonably easy to service.

By Rodney Champness

f we look at the circuits of early 1920s receivers we see that triodes were used to amplify the audio signals, with 1:3 to 1:5 audio step-up transformers between each stage. The triode output stage was then coupled to an output transformer which in turn fed the loudspeaker.

Click for larger image
This view shows the fully restored audio amplifier. Note the new capacitor can at the back, between the two transformers.

In cheaper receivers, the limited output from the triode output stage often fed a high-efficiency, high-impedance horn speaker. These speakers looked beautiful but the audio quality left a lot to be desired.

Certainly until well into the 1930s, the audio reproduction that was obtained could hardly be called "high fidelity" (or hifi). Even in 1935, "Modern Radio Servicing" by Alfred Ghirardi quoted high fidelity as the reproduction of the frequency range from 50 to 7500 cycles at 5% distortion. That’s truly dreadful by today’s standards.

A typical "high-fidelity" amplifier of the 1930s still used triodes in all amplifying stages plus an output transformer. The output transformer matched the high impedance of the triode push-pull output stage to a level suitable for the speaker(s). In addition, some amplifiers also included a push-pull audio driver transformer to act as a phase splitter and driver to the triode output stages.

Even when tetrode and pentode output valves became common, the highest quality audio was still obtained from triodes. Negative feedback also became common during the 1930s. This involved taking a proportion of the output from the secondary of the audio output transformer and feeding it back in anti-phase to an earlier stage in the amplifier.

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