murphy: 4-Valve Screen Grid Portable - Technical Review

Outstanding Range and Selectivity with a Single HF Stage.

A casual glance at the exterior of this receiver is sufficient to mark it as something a little out of the ordinary. The disposition of the controls for instance, with a projecting ledge upon which to rest the hands when tuning, is at once evidence of careful attention to detail on the part of the designers. No carrying handle is provided, as the makers feel that this would spoil the general appearance, but recesses for lifting are cut under the base of the cabinet at each side, and their position is such that the hands automatically fall under the centre of gravity of the set. A waterproof cover complete with a carrying handle and straps fitting under the base is being designed and will be sold as an extra so that the set could be equally well described as a " Portable," "Transportable " or " Table Model." On second thoughts one might be inclined to delete the term " Transportable," as this generally suggests weight, actually, the Murphy portable weighs only 32 pounds, which is less than many so-called " portables."

The Circuit.

The circuit, which comprises a single-screen grid stage of H.F. amplification, reacting leaky grid detector, and two transformer-coupled LF stages, does not contain any strikingly unconventional feature, but rather more than the usual care has been taken to extract the last ounce of efficiency from each. component. Through the courtesy of the directors, we were afforded the opportunity of seeing the sets in course of construction, and of inspecting the apparatus for checking the electrical constants of HF circuits before assembly. Every factor from the frame aerial circuit through the always troublesome detector stage to the output from the power valve has been the subject of exact measurement, and where compromise has been found necessary this has been made on the foundation of concrete fact.

The HF stage is coupled by the parallel choke feed method, the tuned anode circuit being connected between the grid and filament of the detector valve. The frame aerial and anode circuit condensers are ganged, and the HF properties of both circuits are so adjusted that the amplification over both long and shortwave ranges are practically constant. Every precaution has been taken to exclude all couplings between the circuits other than the residual capacity of the valve, which is taken into account when setting the ganging of the two circuits.
The screening material, used in the chassis is tinned iron, and additional precautions to prevent stray coupling include insulation of the tuning condenser spindles, screening between the contacts of the wave range switch, the elimination of common earth wiring, and decoupling of the bias to the screen grid valve. These precautions have borne fruit in another direction in that threshold howl, which is frequent trouble in portables, particularly on the longwave range, has also been eliminated. Unless great care is taken with the detector stage all the precautions made with a view to accurate ganging of the HF circuits are nullified. The working grid filament capacity of the detector, which is in parallel with the tuned anode circuit, must be kept within well-defined limits and of as small a value as possible. To this end, a fixed by-pass condenser and reaction coil of small inductance is used, and control of the reaction is obtained by rotating the reaction coil. The loading effect of grid current has been allowed in designing the tuned grid circuit. Positive bias for the detector valve is derived from a fixed potentiometer across the filament circuit, and the anode voltage is reduced to the required value through a decoupling resistance and condenser.

The valves used in the first and second LF stages are Osram HL-210 and P-215 respectively, and the characteristics of the coupling transformers have been adjusted to compensate for deficiencies in the loudspeaker response. A modulated source of HF is used in conjunction with a completely assembled set in making this compromise so that all factors such as sideband cutting, and box resonance are included in the final result. In this way, the objectionable resonance in the vicinity of 150-200 cycles so common in portable sets has been eradicated, and speech is free from unpleasant " boom." The middle and upper registers are well represented, and the general impression is one of crispness and clarity.

The Controls

Mention has already been made of the convenient disposition of the controls. The wave range switch in the extreme left is smooth in operation and does not produce any sound in the loudspeaker when changing over with the set in operation. Next to it is the main tuning knob for the slow-motion ganged condensers. The condenser scale is calibrated in wavelengths and is viewed through a small window carrying a horizontal hairline.

Broadcast Receivers Reviewed

The reaction control is smooth and free from backlash or threshold howl. On the extreme right is the combined on-off switch and volume control. This consists of a switch for the detector and LF valves and a filament resistance in the positive filament lead of the HF valve.

The performance of the set both as regards range and selectivity fully justifies the care taken in designing the HF circuits. On longwave, there is a commendable absence of mush, and the range is distinctly above the average. No difficulty was experienced in tuning in ten stations where only, six or at the most seven are usually available, and Konigswusterhausen could be received at unusually good strength clear of 5XX and Radio Paris without making full use of the directional properties of the frame (a ball-bearing turntable is a standard item of the equipment).

In the space of half an hour after dark, twenty-four stations at program strength were tuned in on the 200-600 metre band. This is a conservative estimate as several strong carrier waves were passed over which did not happen to be modulated at the time.
At five miles. from Brookmans Park there is not the least difficulty in separating the twin transmitters, while in North-West London at a distance of 10 miles, London National (261 metres) can be limited to a band 7 metres wide, and London Regional (356 metres) to a band 20 metres in width. To obtain these figures the frame aerial must, of course, be set at a minimum, but the performance is exceptionally good for a single HF stage and only two tuned circuits. As further proof of the range, it may be mentioned that Langenburg (473 metres) could be received consistently in daylight.

The photograph of the back of the set shows that the valves are readily accessible, and that economical use has been made of the space allocated to batteries. The HT battery has only two sockets, which greatly simplifies replacement, and the internal resistance would have to rise to 5,000 ohms to produce motor boating-a value unlikely to be reached in practice. The battery is rated for 12 mA. discharge, so that the normal anode current of 8 to 9 mA. is well within its capacity. The measured total HT current of the receiver tested was 8.0 mA. Further evidence of careful attention to detail is provided by the mounting of the LT battery. This is insulated from the woodwork by moulded ebonite guides and a paxolin sheet which is removable for cleaning so that the possibility of acid creeping along the bottom of the cabinet is considerably reduced.

During the course tests, more than 80 miles were covered in a not-too-comfortable car without any detriment to valves or wiring.
To the student of portable set design, this receiver should prove of special interest, as it demonstrates conclusively that a single HF stage coupled with reaction in the detector can be made, by careful design, to give more than adequate range and selectivity without the mush and background noise which is frequently associated with two HF stages.
The price of this receiver complete with valves and turntable but without a waterproof carrier is £17 17s., and the makers are Murphy Radio, Ltd., Broadwater Road, Welwyn Garden City, Herts.

Wireless World September 6, 1930, Pages 127,128.

 

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