Radio Broadcast (May 1928-Apr 1929)

The Isotonic Screen-Grid By DUDLEY WALFORD >uper ESSENTIALLY, there are very few different types of radio receivers, and of these the super-heterodyne is unique in that it is the only one in which the number of circuits which must be tuned to the frequency of the received signal is not made greater with an increase of amplification. In an ordinary tuned radio-frequency receiver, if we wish to obtain more gain, we have to provide additional tubes and tuned circuits, and in operating the receiver it is necessary that these circuits always be adjusted to resonance with the wavelength of the particular station we desire to receive. In a super-heterodyne most of the amplification takes place in the intermediate-frequency amplifier which always operates at the same frequency and therefore, does not have to be adjusted when receiving signals of different frequencies. This is one of the major advantages of the super-heterodyne receiver, and one of the most important parts in such a receiver is the intermediate-frequency amplifier, for upon its characteristics depend the sensitivity and selectivity of the set. Accurate matching during manufacture of the transformers in an intermediate-frequency amplifier, is not very difficult if the intermediatefrequency is low, say 30 to 60 kc. On the other hand, the disadvantage of using a low intermediate frequency in an ordinary super-heterodyne is due to the phenomenon of so-called "repeat points" by which it becomes possible to tune in most local stations at many points on the dial. A super-heterodyne can be made essentially "one spot" by the use of a high intermediate frequency, but at such frequencies the effect of tube capacities, etc., becomes important and accurate matching of the intermediatefrequency transformers during manufacture is not always possible. When using a high intermediate frequency it is of advantage, therefore, to so arrange the transformers that they may be manually adjusted to the point of maximum sensitivity after the receiver has been completely constructed. Such transformers are used in the H. F. L. Isotone receiver described in this article. The following paragraphs will discuss in more detail the technical characteristics of this set. Fundamentally, the Isotone is a standard THE ISOTOHE IH A PHONOGRAPH-RADIO CABINET This receiver was designed for use either as a radio set or a phonograph amplifier, and, when installed in a cabinet of the type illustrated, full advantage is ta\en of its dual entertainment value screen-grid super-heterodyne utilizing nine tubes. There is an additional tube which is used when the instrument is employed for phonograph reproduction. The ten tubes of the set are distributed in the following manner: one 201Atype first-detector tube, one 20iA-type oscillator tube, three 222-type intermediate-frequency tubes, one H2A-type second-detector tube, one 1 i2A-type phonograph-amplifier tube, one I I2Atype first-stage a.f. tube, and two lyiA-type push-pull a.f. tubes. The set itself is composed of four main units, namely, the front tuning unit, the screen-grid intermediate-frequency amplifier, the audio CT'HE H. F. L. Isotone described in this ■*■ article is very different from the standard design of super-heterodyne hit. Whereas the usual set of this type requires many long tedious hours for its construction, the ten-tube Isotone may be completely assembled and placed in operation in less than one hour! This is made possible by the use of wired units which the set-builder fastens to the chassis and wires into the circuit. The design of the set is such that it is almost impossible to make a mistake, and the necessary circuit adjustments are easily accomplished. The Isotone is also efficient when used as a phonograph amplifier. — The Editor. frequency amplifier, and the control box. The wiring and testing of these four units is done at the factory; each piece of each individual unit undergoes several tests and then the entire unit is tested. One of the main features of the receiver which is not apparent from the schematic diagram is the operating frequency of the intermediatefrequency amplifier which is 475 kilocycles. Most set-constructors are by this time aware of the fact that such a frequency allows the receiver to be tuned as a "one-spot" instrument and does away with many of the annoying repeat points on the dial. When such a high frequency is used, it is absolutely necessary, as mentioned previously, that the intermediate-frequency transformers be furnished with a means of compensating the various tube capacities and the capacities of the wiring in the receiver. The tuning of each transformer in this set is accomplished by two condensers — one of these, a small mica condenser, Q, having a fixed value of 0.0001 mfd., is connected permanently across the secondary of each intermediate-transformer, Ti, T2, T3 and T4, and the other, a small variable condenser, C2, having a variable capacity of 0.000025 mfd., is connected in shunt with the fixed condenser, Q. This system of manually tuning the four transformers allows one to adjust the intermediate-frequency amplifier easily to the point of maximum sensitivity and selectivity. In theshield compartments of the intermediatefrequency amplifier are the four transformers, their associate tubes, sockets, resistors, tuning condensers and twelve 0.5-mfd. by-pass condensers. The twelve by-pass condensers are of extreme importance in the proper operation of the amplifier. While their use increases the cost, the results seem to justify the expenditure, for the operation of the amplifier is perfectly stable, and oscillations cannot be produced under any normal operating condition. Immediately to the right of the screen-grid amplifier we see the completely shielded audio section of the Isotone. This consists of four transformers, four sockets, a by-pass condenser, a series resistor and the necessary input and output tip jacks. The first transformer, T5, in the ampli '74