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Vintage Radio

A look at the Radiola Model 573-MA Receiver

By Maurie Findlay, MIE Aust, VK2PW

Before the introduction of television to Australia in 1956, radio and gramophone records were a prime source of entertainment. Typically, a household would have a radio and maybe a gramophone in the lounge room and the family would gather in the evening to listen to the radio or records.

With the war some years away and economic conditions improving, the dream of having several radios in the household became a reality for many. In addition to mains receivers, there were battery-powered portable sets but these were expensive to run and a good many did not work all that well.

The Radiola Model 573-MA operates from mains power and requires only 40W. Furthermore, this set has a very effective inbuilt antenna and can be shifted around to wherever there is a power outlet without having to install an antenna wire. This could be the kitchen, the bedroom or the outside workshop.

The set was manufactured by Amalgamated Wireless (Australasia) Ltd and the valves by their associated company, AWV. The valves were new to Australian technicians at the time and were much smaller than their predecessors which had octal and other plastic bases. They were of all-glass construction, with seven pins at one end providing all the connections.

Circuit details

Click for larger image
Fig.1: the circuit is a fairly standard superheterodyne design with five valves: a frequency changer (V1), an intermediate frequency (IF) amplifier (V2), a detector/audio amplifier (V3), an audio power amplifier (V4) and a rectifier (V5).

Fig.1 shows the circuit diagram of the set. It’s a superheterodyne design with five valves: a frequency changer (V1), an intermediate frequency (IF) amplifier (V2), a detector/audio amplifier (V3), an audio power amplifier (V4) and a rectifier (V5). It’s pretty much a standard line-up for medium-wave receivers designed at the time.

There are a few special features about the design. These include a ferrite rod antenna, a neutralising circuit for the IF amplifier, simple rather than delayed AGC (automatic gain control) and a negative feedback circuit with associated treble cut and boost.

The frequency changer is a 6BE6 which has a simplified geometry in order to fit all the connections within the 7-pin limitation (the screen grid serves as the plate for the local oscillator). This valve actually provides more conversion gain when used on the broadcast band than some earlier octal based types.

Intermediate frequency (IF) amplification at 455kHz is achieved using a 6BA6 which is a variable-mu pentode with AGC applied to the grid. It also has the potential for higher gain than earlier octal-based valve types. Again, looking at the circuit, early versions of the set used a cathode bias resistor without a bypass capacitor in order to reduce the gain. Later models included the bypass capacitor as well as a neutralising circuit.

The detector/amplifier stage (V3) is a 6AV6 which has two diodes and a triode in the one envelope. One diode detects the 455kHz intermediate frequency signal and at the same time provides the AGC voltage. The other diode is not used and is simply connected to earth. The triode section provides substantial audio gain and is a commonly-used circuit. The grid return resistor (R9) is 10MΩ while its plate load resistor (R12) is 0.22MΩ.

The the 6AQ5 valve (V4) is the familiar beam-tetrode in miniature form. It provides gain and the audio power to drive the loudspeaker. R14, a 47kΩ resistor in the grid circuit, is there as a precaution against parasitic oscillations at frequencies outside the audio range. The valve must be operated in a linear mode for low distortion and negative bias is provided via R17, a 150Ω resistor in the supply line.

V5, a type 6X4, rectifies the AC output of the transformer to provide 240V DC for the plate of the 6AQ5. It also provides, via dropping resistor R16 (5kΩ), 165V DC for the screen of the 6AQ5 and the plates of the other valves.

A special feature of the 6X4 is that it has insulation between the heater and cathode elements, designed to withstand the high-tension (HT) voltage. The heater can be operated from the same supply as the heaters for the other valves, ie, with one end connected to earth.

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