Radio age (Jan 1927-Jan 1928)

RADIO AGE for October, 1927 Jewelers' Time Signal Amplifier incorporated in Superheterodyne receiver Bringing the Old Super Up to Date HARKING back to the old days of the UV-1716 transformer — when that item was the only available one at the disposal of the broadcast listener that could be used as an intermediate transformer in super-heterodyne construction, and following this circuit to the present day, it is apparent, above all other things, that a proper intermediate amplifier is the very key to the whole super-heterodyne situation. Realizing this and also taking into consideration the added density of broadcast station population it is not hard to see that receivers of this type constructed sometime ago were not built to function to the point of present day standards, especially as far as selectivity and sensitivity — which means DX reception — is concerned. Two principal factors are responsible for this, first the situation that concerns intermediate frequency transformers, the other the broadcast situation. Up to the present time the builder of a super-heterodyne was always forced to rely upon individual units for the construction of the intermediate amplifier — usually three or four transformers— most of the time selected out of random stock — and at other times on transformers which were sold as being peaked but which when assembled and in operation had entirely different characteristics in the receiver than when they were matched. This resulted not only in a lack of selectivity and sensitivity, far from the real possibilities of a super-heterodyne circuit, but in many cases resulted in a tone quality with which discriminating fans were not satisfied because of the cutting of the side bands owing to improper peaking. It is generally conceded by all of those who have studied the various characteristics of receiver designs on the market that the super-heterodyne principal of reception al lows not only the sharpest of tuning and the ability to reach out farther than any other circuit, but to make these possibilities a reality requires 100% efficiency in each of the circuits that comprise the whole. On the other hand, a super-heterodyne that served our purpose two or three years ago with broad intermediates, is no longer able to cope with present day broadcasting conditions because where we had one station three years ago we probably have ten today. The old set with broad intermediates was sharp enough for the condition of those days but now the set probably will not even be able to break through where broadcasting stations abound, especially in localities like New York and Chicago. There is also another item that has not been mentioned before and that is the increase in power used by broadcasting stations and this in congested areas has further handicapped the older receiver while, on the other hand, this condition with a really sharp set of intermediates if anything has been "pie" for the super built for present day broadcast reception. We now find upon the market a Jewelers' Time Signal Amplifier which adapts itself effectively to a super-heterodyne intermediate amplifier and through the use of which all of the difficulties experienced in this particular section of the receiver are overcome. This Jewelers' Time Amplifier consists of three stages of R. F. amplification and a detector — a unit designed primarily for the jewelry trade in receiving the time signals from station NAA — Arlington —whose wavelength is 2677 or 112 K. C. This unit is constructed to automatically tune in that station and no other — sharply peaked to NAA frequency and with a band so narrow as to prevent interference from any other station. As one fan remarked recently, this amplifier used as an intermediate unit in a super in reality amounts to a "band-pass amplifier." How this works out can best be realized when we stop to consider that the signal band in this amplifier is but 10, sometimes 7 K. C. wide. As all broadcast stations are separated at least by 10 K. C. this unit is by far the most selective and sensitive that has been on the market. The amplifier is peaked and wired and ready to hook into the circuit. The four component circuits of the amplifier are peaked as one unit instead of four, in other words, our intermediates can be treated as one unit rather than a number as previously. Large air core transformers with a tremendous amplification gain are used in every stage — the secondaries all being tuned with matched fixed condensers. Each stage is individually shielded and the entire assembly is housed in a copper and brass catacomb which does away absolutely with any possibility of interaction of circuits or the pick up of parasitic noises. The amplification gain as measured in laboratory tests exceeds considerably that of any amplifier that can be built of individual parts for here is found an amplification gain of 10 per stage or a total amplification factor of 10,000 for the unit with the unit non-critically adjusted. This Jewelers' Time Signal Amplifier be: ing absolutely complete in itself (with the exception of tubes) contains all of the parts that are Usually found in an intermediate frequency amplifier. In the assembly in addition to the four large air core transformers are four tube sockets, four fixed tuning capacities (one across the secondary of each transformer), two bypass condensers, of 1 mf. capacity and one fixed condenser of .002 mf. capacity.