• Year
  • 1938
  • Category
  • Broadcast Receiver - or past WW2 Tuner
  • Radiomuseum.org ID
  • 78751

Click on the schematic thumbnail to request the schematic as a free document.

 Technical Specifications

  • Number of Tubes
  • 9
  • Main principle
  • Superhet with RF-stage; ZF/IF 465 kHz
  • Wave bands
  • Broadcast, Long Wave and Short Wave.
  • Power type and voltage
  • Alternating Current supply (AC)
  • Loudspeaker
  • -Loudspeaker incorporated, but system not known.
  • Material
  • Wooden case
  • from Radiomuseum.org
  • Model: A52 - Murphy Radio Ltd.; Welwyn
  • Shape
  • Tablemodel, low profile (big size).
  • Dimensions (WHD)
  • 24 x 15 x 9.5 inch / 610 x 381 x 241 mm
  • Notes
  • On short waves the Murphy A52 is a double conversion superhet with IF 465 kHz and 3,1 MHz. Broadcast and LW the A52 is a "normal superhet" with a RF-pre amplifyer. SW = 16,1 to 50 m (18,75 to 6 MHz).
    Pushbuttons for 7 freely programmable LW/BC frequencies and for 7 fixed spread SW bands (with a separate pointer at the left side of the dial.
    Hill states 9 tubes (8 + Rect. + mag eye)?
    German: Die Programmtasten des Gerätes lassen sich auf Lang- und Mittelwelle frei belegen; je Wellenbereich stehen 7 Plätze zur Verfügung; insgesamt lassen sich damit 14 Sender programmieren. Auf Kurzwelle lassen sich über die Programmtasten 7 fest eingestellte Bänder wählen, die dann gespreizt werden und über einen Extra-Zeiger am linken Rand der Skale durchsucht werden können. Im Prinzip läßt sich aber auch jeder beliebige Teil im Kurzwellenbereich spreizen, wenn auch etwas umständlich. Auf Mittel- und Langwelle dient die zweite Mischröhre AC/TH1 zusammen mit der DD41 für das AFC-System (was bei Murphy Automatic Tuning Correction, ATC, hieß). Das wird nur im Zusammenhang mit den Programmtasten verwendet
  • Net weight (2.2 lb = 1 kg)
  • 24 kg / 52 lb 13.8 oz (52.863 lb)
  Murphy A52 Pamphlet. 2467 KB
  • Documents regarding this model

 Collections | Museums | Literature

 Forum

Forum contributions about this model: Murphy Radio Ltd.;: A52

Threads: 1 | Posts: 1

Murphy A52

An Efficient Receiver with a Well-Designed Automatic Tuning System.

The specification of this receiver includes the best points of many previous Murphy designs and is made singularly attractive to the non-technical user by a push-button tuning system which is one of the best we have so far tested. The exterior of the set gives no clue to the complex functioning of the tuning system, and the controls are extraordinarily simple and foolproof. Operation of the wave range switch automatically changes the identification tabs alongside each push-button and also lights up the appropriate tuning scale when the manual tuning control is required. It also changes over the tuning control mechanically from the main condenser to a separate variable inductance tuner on short waves by transferring the friction drive. Seven stations may be selected on medium waves and an equal number on long. A happy thought was the inclusion of "Weather London" in the latter group as adjusted at the works. On short waves, the pushbuttons are used to select the seven principal broadcast and amateur bands, each of which is expanded to the full length of the tuning scale. When changing from one band to another the main tuning pointer is stopped automatically at a position approximately in the centre of the band.

There is no possibility of causing damage by pushing a button or turning the wave range switch without thinking.

One should, however, refrain from turning the manual tuning knob before the “manual" button has been pressed. With automatic tuning, there is a brief whirr of the motor mechanism, a click and a station or short waveband is ready to use. Its identity can then be ascertained by a glance at the push-button panel. The contact brushes are well designed, and medium and longwave groups are arranged on separate tracks on the outside and inside edges of the contact rail. The shortwave contacts occupy slots in the rail between the inner and outer tracks. Adjustments can be made after removing a semi-circular "window" in the back panel of the set.

Outstanding Short-wave Performance

Good as is the performance on medium and long waves, it is the short waves that must take first place in any assessment of the merits of this receiver. Not only does the electrical band spreading make their tuning as easy as that of mediumwave stations, but from the point of view of volume and signal-to-noise ratio, the shortwave performance approaches much nearer to mediumwave standards than we had thought possible in a commercially produced receiver. In this respect, the A52 is an advance on the A36, which at the time of its introduction was equally remarkable for its shortwave performance. Not only is the sensitivity higher but selectivity and image suppression show a corresponding improvement.

On medium and long waves clean and whistle-free reception of every worthwhile program seems to have been the aim of the designers rather than record-breaking sensitivity and selectivity. It is recommended that the set should be worked at a level that avoids overloading of the frequency changer by the local station, and a preset control has been provided at the back of the chassis for this purpose. Selectivity on medium waves clears the London Regional station outside 1½ channels on either side of its normal setting when using the set at a distance of fifteen miles from Brookmans Park. On longwave, there is appreciable interference from both Droitwich and Radio Paris on the Deutschlandsender between these two stations.

Quality of reproduction is clear with just the right amount of top, and no attempt has been made to compensate for the limitations of a table model by the introduction of a false bass. Harmonic distortion is evident only when the volume control is turned up too far, which it can be on most stations.

The two triode hexode valves are the first to catch the eye in the circuit. The one following the RF stage converts to 465 kc/s on medium and long waves and to 3.1 Mc/ s on short waves. Unlike the A36, the intermediate frequency in the A52 is fixed, and bandspread tuning is effected by changing slightly the inductance of the shortwave oscillator coils. Approximate tuning to the middle of each waveband is carried out by the main tuning condenser under push-button control.

The second triode hexode acts as a frequency changer from 3.1 Mc/s to 465 kc/s when the wave range switch is set for shortwaves, and on medium and long waves as control varve for the oscillator in the automatic frequency control circuits. Bias for this valve is derived from two diodes connected to the output IF transformer, and its magnitude is determined by phase differences in that transformer, depending on whether the intermediate frequency is above or below the resonant frequency of the tuned circuits.

A double-diode-triode provides delayed AVC for the IF and both frequency changer stages, in addition to its other functions of signal rectification and first stage AF amplification.

Feedback in the RF Stage

The RF amplifier is not controlled as it is of the non-variable-mµ type. It is a television valve with an exceptionally high gain and a good signal-to-noise ratio at high frequencies. The high magnification is something of a liability on medium and long waves and may result in whistles if the frequency changer is overloaded. Accordingly, the overall gain of this stage is reduced by negative feedback on the two longwave ranges. The control consists of a variable cathode resistance, without a by-pass condenser, which is automatically short-circuited by the wave range switch on the short-wave range.

Negative feedback is also applied in the output stage in such a manner as to reduce the valve resistance as well as harmonic distortion. To suppress heterodyne whistles a filter circuit has been included in the feedback line, and this method has distinct advantages over a general depression of the high note response by means of the conventional forms of tone control. The usual variable resistance capacity filter has been included across the primary of the output transformer, but this is not the kind of set which makes frequent calls upon its services. Short, medium, and longwave stations are all alike in giving good programs with the minimum of background noise, and throughout the course of our tests we were always able to enjoy the full response of which the loudspeaker is capable.

The Wireless World, March 2nd, 1939, Pages 10-12.

Gary Cowans, 10.Mar.22

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