Dynatron Universal Radiogramophone U53

Extracted from Wireless World March 30, 1932, Pages 326 & 328.

The choice of a suitable receiver is always a source of embarrassment to the DC mains user, for he is often afraid that a sudden changeover of his lighting supply to alternating current may render his set useless. It is interesting to find, therefore, a receiver that, merely by changing a small and inexpensive power unit, can be operated from either type of current, for such a set can never be put out of date by any change in the nature of the electric light supply.

A glance at the circuit will show that the heaters of the valves, which are all of the indirectly heated cathode type, are separately wired to a multi-way connector. For AC working, the insertion of the appropriate plug automatically connects all the heaters in parallel and to the mains transformer secondary; with D.C. mains, however, a different plug is used which connects the heaters in series with one another and with a suitable value resistance for dropping the mains voltage. The valves used are of the AC type, even for DC working, so that the results obtainable are independent of the supply.

Save for the use of a push-pull pentode output stage, the circuit appears to follow normal practice, and two HF stages precede the detector. One would be pleasantly disappointed if one expected only normal results, however, for the performance is distinctly above the average. The selectivity, in fact, is extraordinarily high for a set of this type and is nearly as good as that of many superheterodynes. At a distance of only nine miles from Brookmans Park, for instance, it is possible to receive Algiers clear of the London Regional; as the frequency separation is only 18 kc., this speaks volumes for the performance of the set.

The Tuning Circuits.

This high selectivity is due more to careful design than to any inherent virtue of the circuit, and examination of the chassis shows that the tuning coils are of large diameter and wound with heavy gauge wire. The intrinsic selectivity of each circuit, therefore, is above normal; in order that advantage may be taken of this higher selectivity, however, it is necessary so to connect the coils in the circuit so that they are not damped, and it will be seen that the anode connections of the HF valves are tapped well down the tuned circuits.

In spite of the fact that the detector grid connection is also tapped down its tuned grid coil, the damping of this circuit is inevitably higher than that of the others It is for this reason, therefore, that reaction is fitted, for by its use the unwanted damping can be removed when interference is present. The input band-pass filter is of unconventional design, for its coupling is a combination of common inductance coupling and capacity coupling at the high potential ends of the circuits. Naturally, this does not give a constant bandwidth over the tuning range, but, in conjunction with the other tuned circuits, it gives an even more important property, an approximation to constant selectivity.

Each tuned circuit is individually screened; the detector valve, with its anode circuit filter and the radio-gramophone switch, is contained in another screening box, while the two HF chokes for the tuned grid couplings are not only of the astatic type but are also completely screened. Each HF stage is thoroughly decoupled, and that the means taken to avoid instability are effective is vouched for by the fact that a considerable degree of reaction is necessary to provoke self-oscillation.

As might be expected with variable-mu valves, the radio volume control takes the form of a potentiometer controlling their grid bias. A separate potentiometer is used for gramophone volume control, and it is ganged to the radio control so that both are operated by a single knob. Both on radio and gramophone, their operation is very sweet and distortionless, and an ample range of control is afforded. The use of pentodes in push-pull for the output stage is unusual, and they are rated to deliver some three watts to the moving coil loudspeaker. Anti-parasitic resistances are fitted in the anode circuits, and a tone control is provided by a variable resistance and a condenser connected across the primary of the output transformer.

A good point which is worthy of more general application is the inclusion of jacks in the anode circuits of both the detector and the output valves, so that a milliammeter may be inserted as a check on the operating conditions and the state of the valves.

The whole AC smoothing equipment is built into the chassis; three chokes are employed with the usual condensers and are effective in eliminating mains hum. On DC two additional chokes are included in the power unit; a total of five chokes is used, therefore, and as might be expected, the set is as silent as on AC.

The sensitivity is ample for all normal purposes, and on tests with rather a poor aerial, it was never necessary to have the volume control at maximum. The quality of reproduction reached an exceptionally high standard, and this is no doubt partly due to the large dimensions of the cabinet, which permit the lowest notes to be effectively radiated. It is pleasing to find, however, that despite an ample bass, the high notes are well in evidence, and there is no trace of boominess. Openings are cut in the sides of the cabinet to prevent box resonance, and the speaker is mounted on a board, of acoustically dead material.

The cabinet work is not only of pleasing appearance but is very solidly constructed; it would be an improvement if the lid were made to open more widely, for if the set be used in a dark corner, the changing of records and needles is more difficult than it need be.

On DC mains the voltage-dropping resistance for the valve heaters, which are mounted on the back of the cabinet, runs at a fairly high temperature, and it is in consequence advisable to stand the set some little distance from a wall.

The apparatus is claimed to be of entirely British manufacture, and an inspection of the chassis shows that all the components are of well-known and high-quality British makes, even in the case of parts that are normally hidden by screening material. The components are obviously chosen solely on the grounds of their suitability for the function which they have to perform, and not merely for their cheapness.

Gary Cowans, 07.Jan.24

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