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AERODYNE   SWALLOW, WREN   MAKES OF SET  BLACKBIRD  SWALLOW  CURLEW  MERLIN  EPITAPH

 

SWALLOW

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 leaflet.

"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...

 


Terminology explained

AVC = 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.

WREN

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 design.

 


Terminology explained

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 down.

 

 

 

 

 

VINTAGE RADIO world: SIXTEEN YEARS OF WEB PRESENCE