The same basic cabinet as the 'Curlew', but with different cross-bandings. This
housed an AC only five-valve chassis and was the most expensive set in the
"The Aerodyne 'Swallow' is a seven stage
superheterodyne. It embodies the very latest developments in both circuit and
valve research, and has every modern refinement including sensitivity,
selectivity, automatic volume control, tone control, noise suppression between
stations and automatic station rejector.
Special attention has been paid to tonal reproduction,
which is superlative. The cabinet in beautifully veneered Walnut of exquisite
design is in the usual good taste for which Aerodyne is justly famous.
Under practically any conditions this set will receive
stations worth hearing, British or Continental, with quality, purity of tone and
volume unsurpassed by any other receiver."
There is no doubt that the subjective sound quality of the earlier Aerodyne
'Swan', with its 10" Rola M.E. speaker, was pleasant to listen to. The
'Swallow', with improvements such as tone control, AVC and a neat inter-station
noise suppression circuit, would probably have sounded very good indeed by the
standards of the time.
The comment 'seven stages' can be slightly misleading. Five valves were
employed, one of which would have been the rectifier, so should not really be
counted as a 'stage'. A single octode valve was used as a frequency changer, and
a double-diode-triode for signal detection, AVC and AF amplification. Adding the
rectifier stage allows the claim of seven stages to be justified...
= automatic volume control. Nowadays called automatic gain control
(AGC), it is a system to prevent powerful or nearby stations blasting out as the
set is tuned through them. The gain of one or more of the valve stages is
automatically adjusted by the use of Vari-Mu valves and a bias voltage derived
from the rectified RF signal. The larger the rectified signal, the greater the
bias applied to the Vari-Mu stages, thereby reducing their gain commensurately.
In its simple form, it leads to insensitivity as it is always in operation, even
on distant, faint stations, so a 'delay' is built in to the designs to stop it
operating on low strength signals.
The triode output valve would have placed a limit on the volume
obtainable with this set, but within its limits the quality of sound would have
been good. The triode detector means it was either of leaky-grid or anode-bend
Anode-bend and Leaky-grid detection
= Here is a
simplified explanation of the terms. Anode-bend rectification made use of the
non-linear portion of the characteristic curve of a valve, the valve being
biased either to work on the upper or lower bend in the curve. This resulted in
unequal amplification (crushing) of either the positive-going or negative-going
parts of the RF signal. It worked well enough: the main disadvantage was the
degree of distortion that could occur.
Leaky grid detection used the AF amplifier valve grid/cathode
(or filament, in this case) as a diode anode. A grid-leak resistor of a high -
and critical - value allows the charge building up on the grid to 'leak' at a
slow rate. In conjunction with a grid capacitor, a time-constant circuit is
created that removes the high frequency RF component of the signal, leaving the
AF signal behind. This is better than the anode-bend detection system in terms
of quality, but still leaves something to be desired! Basically, few
components were needed for either of these two detection systems, keeping production costs