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.
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.