Radio Broadcast (May 1928-Apr 1929)

The Receiver Combines Simplicity and Beauty The "Vivetone 29n Receiver THE receiver which Mr. Goodwin describes in this article is a t.r.f. set which performed in the Laboratory in a fashion to indicate that its designer is not prone to exaggeration. There are several novel features; one is an automatic regeneration control, consisting of a resistor which changes its value as the set is tuned, thus adjusting the C bias on the r.f. tubes and keeping them from oscillating at any dial setting. Another is an a.c. voltmeter which is mounted on the panel so that the operator can tell at all times the voltages across his tubes. If the constructor builds one of the amplifier-power supply units the author has developed, he will have in il a regulating device which will enable him to keep this voltage at its proper value regardless of line voltage fluctuations. — The Editor. SENSITIVITY, selectivity, quality of reproduction, beauty in appearance, and simplicity of operation — these are the five paramount requisites of a modern receiver. Every discriminating radio constructor or enthusiast must consider each of these features separately and in connection with each other when he decides what receiver to build, buy, or operate. Some of these features are often secured at a sacrifice of others; as in other phases of life one cannot get something for nothing. The "Vivetone 29" is a receiver in which the effort has been made to incorporate all these features to as high a degree as possible without too great a compromise. In other words, it embodies a circuit that has considerable radiofrequency gain, that is simple to operate — there are only two controls — and yet is sufficiently selective to cope with modern broadcasting conditions. More will be said about the sensitivity and selectivity later when the writer describes his own success with the receiver in a location which is none too good for dx reception. It is enough to state now that the selectivity is sufficient to enable one to tune through many local stations with a minimum of interference. By R. F. GOODWIN and that the sensitivity is such that distant stations can be picked up with a great deal of ease. DESIGN FEATURES THE fact that the set contains three stages of r. f. amplification, each separately shielded and constructed of the best "low loss" apparatus now on the market, may account to the technical reader for the set's gain. With the addition of the detector input, which is tuned, the receiver has four tuned circuits, each working with a minimum of regeneration so that the order of selectivity is rather high. There are two novel features, one of which has not been seen heretofore so far as the writer is aware, and the other — an a. c. voltmeter on the panel — is omitted from the vast majority of otherwise well designed sets. The novel feature which impresses the writer most is a simple but effective method of automatically controlling the regeneration in the r. f. stages. This is done by automatically varying the bias of the grids of the r. f. tubes as the set is tuned to various frequencies. It is well known that all t.r.f. receivers tend to oscillate badly on the high frequencies (shorter wavelengths) and that many such receivers have included in their mechanism a resistor which changes the plate voltage, or C bias, of the r. f. tubes so that actual oscillation does not take place. Such devices are always manually operated, so that at each setting of the tuning condensers it becomes necessary to adjust the regeneration control — often labelled a volume control — to the point of best operation. In this receiver all such adjustments are made automatically by attaching a 2000-ohm variable resistance, PP-2000 in Fig. 2 and 3, to the shaft of the tuning condensers. It is so adjusted that as the tuning condensers are varied the bias is reduced at the lower frequencies and increased at the higher frequencies. This has the same effect as changing the plate voltages to these tubes, but in the writer's experience is a neater method of accomplishing the same result, i. e., the maintenance of greater stability as a whole. in addition to the automatically changing resistance, another resistance is shunted across 366 the bias adjuster for fine adjustments. This resistance, PP-5000, is located on the panel so that manual regulation may be had when desired. It must be understood that it is not necessary to change the setting of this resistance at all for ordinary operation, but when one is dx hunting he often needs just that small additional control which "brings 'em in." On the panel is an a.c. voltmeter which, after the receiver has been placed in operation, is permanently connected across the heater element of the detector tube. It shows the voltage across that tube at all times, and since this tube in common with all the others is fed from a filament supply transformer the meter's indication tells what the general voltage conditions are. The writer has developed a series of power units for this receiver in which are incorporated voltage regulating devices. It is a simple matter, then, to keep the voltages across the filaments and heaters of the tubes at the proper value at all times. If the receiver is operated with another power device in which there is no provision for voltage adjustments, the operator may obtain such regulation by placing a power resistor in series with his power transformer primary. AUDIO AND POWER UNITS THE "Vivetone 29" receiver proper does not incorporate an audio amplifier, since the writer does not believe that an audio amplifier should be built in the same cabinet with the r.f. portion if the size of the set is to be kept within attractive proportions. Keeping it separate from the tuning elements of the receiver not only gives the builder a choice of audio equipment, but also permits him to have more room for the disposition of his r.f. apparatus. Space in this part of the receiver is most important when one is interested in sensitivity and selectivity. The writer has developed two power amplifier units that can be constructed for the "Vivetone 29." One of these uses a cx-310 tube in the last stage with a Thordarson 210 power pack. This unit will be described in a later issue. Each of these units incorporates a complete audio amplifier and A-B-C-power supply. The home con->