Radio Broadcast (May 1928-Apr 1929)

The Master Hi-Q Receiver in Its Cabinet The "Hi-Q 29"— A Receiver with a Band-pass R* F* Amplifier IN THE design of a modern broadcast receiver it is conceded that quality of reproduction and selectivity are of prime importance. Also, in most cases, a high degree of radio-frequency amplification is a distinct asset, if it can be secured without loss of stability and without affecting the preceding qualifications. A high-gain r.f. amplifier preceding the detector tube increases the sensitivity of the receiver as a whole. Great sensitivity is highly desirable from two standpoints. First, it enables the set owner to receive programs from very distant stations when he feels so inclined, and second, it makes possible quite satisfactory reception from local and moderately distant stations on a very short indoor antenna even in unfavorable locations. Unfortunately, these three prime requisites of a fine receiver, quality of reproduction, selectivity, and sensitivity, are by no means independent of each other. For example, the modern highquality audio transformers now available make possible the construction of a practically perfect audio amplifying system. If a power tube is used in the last stage of such an amplifier and its output fed into one of the better type speakers, the audio amplifying and reproducing system leaves little to be desired. However, this system can only amplify and reproduce what is fed into it by the detector tube, which in turn receives the signal from the radio-frequency amplifier. Hence it is evident that even a perfect audio system cannot provide a high quality output from the loud speaker if distortion is introduced in the r.f. amplifier due, let us say, to excessively sharp tuning, technically known as "side band cutting." In the same way selectivity and sensitivity are incompatible. One of the reasons for this condition is not generally understood, and is even more seldom taken into consideration. The average receiver owner or experimenter bases his judgment almost entirely on the "apparent" selectivity. This is quite natural, in view of the fact that the actual selectivity of a receiver can only be determined by a series of very careful measurements. The apparent selectivity of the Hammarlund Mfg. Co. /~^\LD timers in the radio game who remember the difficulty in getting a lot of current into the antenna without the double humped resonance curves which usually resulted from their efforts may be shocked to learn that a use has been found for such broadly tuned circuits. Since Dr. F. K. V reeland gave his paper on band selector circuits before the I. R. E. we have been waiting for the receiver that would put into practice such ideas, which consist briefly in making use of the "flat top" tuning effect of closely coupled tuned circuits or of the "staggering effect" of several circuits tuned to slightly different frequencies. The present Master Hi-Q receivers employ two band-pass filters — that is, two circuits tuned to the same frequency and coupled together, with the result that the resonance curve is very flat over a range of frequencies controlled by the constants of the circuit and with very steep sides, so that unwanted stations have more than usual difficulty in making their presence heard. — The Editor. ordinary radio set decreases as its sensitivity increases. Therefore, of two receivers having exactly similar "actual" selectivity and one having, say, three times the sensitivity of the other, the set having the higher sensitivity (or amplification) will invariably seem broader or less selective. This principle is very clearly shown in Fig. i. Curve A is the response curve of the less sensitive receiver when tuned to 600 kc. Assuming that no 600-kc. station is on the air at the time no sound will be heard from the loud speaker, as the sensitivity of the set is not great enough to bring the 580-kc. station (which is assumed to be on the air at the time) above audibility. Curve B represents the response characteristic of the more sensitive receiver, and under the above mentioned conditions the 580kc. station will now be heard, since the increased amplification of the more' sensitive receiver is sufficient to bring the signals above audibility. Thus it is quite easy to understand why the more sensitive of the two sets will appear to be less selective, although in reality one is equally as selective as the other. THE ADVANTAGE OF THE SCREEN-GRID TUBE THE new 222 type screen-grid tube with its high amplification factor and extremely low plate to grid capacity would at first glance seem to be ideal for use in r. f. amplifiers. The manufacturers of these tubes state that a voltage step-up of forty or more per stage is obtainable at broadcast frequencies. In addition the plate-togrid capacity is said to be of the order of onefortieth of a micromicrofarad or about one-four hundredth as great as that between the plate and grid of the 201 a type tube. Therefore the appearance of the screen-grid tube, with a capacity so small that neutralization is unnecessary, was welcomed by set designers, and many circuits using them made their appearance. Many of these sets did have enormous amplification, making possible quite satisfactory reception on very short antennas. The selectivity of these sets, however, left much to be desired — so much so that the tube acquired the reputation of causing broad tuning. In planning the set to be described the natural advantages of the screen-grid tube were carefully considered, and the various methods of overcoming the apparent disadvantages were also investigated. Two stages of r.f. amplification were decided upon as sufficient, as they could reasonably be expected to produce an overall voltage gain of more than one thousand. In order to achieve a high degree of selectivity with this amount of amplification some special form of tuning is necessary. The conventional antenna coupler and two interstage t. r. f. transformers were found to be wholly inadequate in the matter of selectivity, although the amplification was good. The tuned plate-impedance coupling condenser and grid leak arrangement specified by the manufacturers of the tube was passed up for the same reason. Calculation showed that it was quite feasible to tune both the grid and plate circuits of these screen-grid tubes. This is one of the marked advantages of this type of tube, since an attempt to tune both grid and 341