A Five Valve “Generic” Superhetrodyne MW and LW radio
The aim of this project was to construct a a five valve mains superhetrodyne for operation on the MW and LW wavebands. The radio is very much in the spirit of articles published in“Radio Constructor” magazine of the 1960s. The circuit is almost a generic layout with various ideas coming from Radio Constuctor and also published designs and circuits from the “Radio and TV Servicing” series. The Mullard valve line up consists of ECH81 frequency changer and local oscillator, EF89 intermediate frequency amplifier, EABC80 detector, agc and audio amplifier, EL84 audio output amplifier and EZ80 HT rectifier. The coils used are the Denco DP Blue series aerial tuning coils and Red series oscillator coils. IF transformers are Radio Spares miniature 455 Kc/s types. Transformers are newly manufactured types, the audio output transformer is from Oxford Electrical Products (OEP), the mains transformer is made by Variable Voltage Technology.
The circuit is very straightforward in most areas, a neutralization network is applied to the EF89 IF amplifier valve following some stability issues. A EF85 was also tried but found to be susceptible to oscillation.
Results may be considered reasonable, a number of areas may warrant circuit changes
1. Audio quality: Seems a little disappointing and not as good as some other AM valve radios. Changes to the audio output stages could be made in the area of negative feedback. Also some audio sibilance is present possibly related to the IF bandwidth and detector circuit.
2. AGC operation: This could be subject to change as the radio does not cope well
with a local AM station and this station appears as images at various points in the
tuning rage. The valves may be out of spec or more likely changes to the AGC bias
may improve local-
Any comments or suggestions regarding this circuit are welcome.