Radio Broadcast (May 1929-Apr 1930)

Record Details:

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RADIO BROADCAST To Audio Fig. 1 tuning forks with the audio-frequency output of the tube. THE USUAL COMMENT Speaking as interlocutor permits me to say that immediately after seeing Mr. Snow's apparatus I went home and repeated his work in a slightly different manner. One thereby gains new understanding of R. A. Heising's remark that "A vacuum tube is something that hasn't anything in it and out of which you get a lot of things you don't expect." One of these unexpected things is the tiresome tuning effect of everything about a short-wave receiver that is moveable or changeable. In most tuners, especially for code reception, this is deliberately made worse by using very small tuning capacity in an attempt to spread the scale. Intelligent use of lumped capacity accomplishes the same thing and in addition washes out some disagreeable effects. Concerning a method of doing this we will now hear from Mr. L. W. Hatry, of Hatry and Young, a radio service organization at Hartford, Connecticut. A Stabilizing Device SINCE the war the short-wave amateur has had trouble with the tuning effect of his regeneration control. The effect has received generous attention but to my knowledge no theory of its possible cause has been used as a weapon in overcoming it. In speculating toward the cause amateurs have suggested inductive and capacitive changes due to moving parts. A marked improvement resulted from making the tickler small and putting it at the low-potential end of the tuned grid circuit. Likewise, improvement resulted from making the coil stationary and controlling regeneration by variation of a "throttle" in the shape of a variable bypass condenser. Thus, the speculations were to a degree confirmed and the effect of the regeneration control reduced until a c.w. beat note did not disappear (above audible pitch) with one degree of movement of the regeneration control but stayed in for ten or more. The circuit for throttle regeneration control used in the majority of short-wave detectors to-day is shown in Fig. 1. Guesses as to the cause of the tuning effect of C2 must have died nascent, for few reached print and the oral discussions seemed to equate to zero. One point seemed accepted, that La, Ci, being the frequency-determining circuit, must suffer a change in constants when C2 is varied. But how? Since the tickler is fixed, coupling changes between the coils seem unlikely. With no change in Li we are left with the probability of such a change in Ci. Capacity effects between the rotor of C2 and other parts of the set can be considered but a simple sheathing experiment eliminates that explanation. Thus we are referred back to the tube for the explanation. The input capacity, Ci, of the tube is in shunt with Ci and, therefore, is of importance. The simplest formula offering an explanation is: Ci = Cgf + Cgp I + 1 This means that Ci can be several times as large as CgP if the load resistance, R0, (See (A) Fig. 3 The General Radio type 557 bandcovering variable condenser. Fig. 2) is several times larger than RP, the plate resistance of the tube. The simple formula applies only to a pure resistance load but illustrates the point. Usually R0 is replaced by a reactive load of which C2 is a part, therefore, the load changes with changes in C2 and we have at hand a plausible explanation of our effect. The increased effect shown by a relatively small increase in CgP lead to other attempts involving the use of a 199-type tube and eventually a 222-type tube as a detector because of low values of grid-plate capacity. The screengrid tube when correctly wired gives less tuning effect of the regeneration control than any other equally well-designed set-up. However, the 222-type tube has disadvantages on short waves since the large plate-to-screen (and therefore to filament) capacity prevents oscillation, sometimes even at 20 meters. A special screen-grid tube might be devised as a short-wave detector and for the further confusion of the innocent. A refinement of Fig. 1 outwits the tuning effect. Fig. 3 gives the diagrams. In (a) the minimum capacity of Ci is made large so that the variations of Ci 7 -„ Swamping "I Capacity B (B) Fig. 4 have little effect on the total capacity. The added capacity can be fixed and attached to the plug-in coil; an old trick that is too little used for band-covering tuners. Likewise it may be built into the tuning condenser as shown in one of the pictures. This condenser has a maximum of 70 mmfd. but the minimum is over one half as great — namely 43 mmfd. In Fig. 3 (b) the input capacitance of the tube is swamped by a condenser on the tube side of the grid condenser and the grid condenser is made as small as 0.000025 mfd. The only other detail to be observed is to keep the grid and plate leads apart. As a test of this scheme such modifications were made in a commercial kit set with an especially bad tuning effect from the regeneration control. The 0.0002-mfd. grid condenser was replaced by the smallest available (0.00004 mfd.), which in itself made an improvement. The effect of a 0.000025-mfd. (25mmfd.) swamping condenser was then tried on the tube side and condenser side of the grid condenser. As expected it had about 4 times as good an effect on the tube side and the tuner became a thoroughly workable one. AS TO TRANSMITTERS Just as the short-wave oscillating detector suffers from frequency changes not undei control of the tuning circuit so does any other oscillator. In the transmitter no frequently varied oscillation control is used: the unit is required to generate a steady frequency and changes cannot be blamed on an altered condenser in the plate circuit. However, changes in frequency can be proved to occur with changes in plate voltage or tube temperature. Changes in plate voltage, EP, result in changes of plate-to-filament resistance, RP. In the formula, RP is important when R0 is small and fixed, both of which are true in a transmitter or test oscillator. Accordingly, variations in Ep vary Ci. Thus, the highcapacity tuned circuits of the present vogue are seen to be good logic as well as good practice. High-capacity tuned circuits have certain uncomfortable disadvantages, especially at high powers, and beside they should not be the only fruit of the seed of knowledge. Input capacity variations can be swamped out in a transmitter as has just been shown in receivers. For instance, in Fig. 4 we have a transmitting application of Fig. 3. This ; circuit has demonstrated its ability but neither it nor the so-called "H-C" arrangements are of much use if the builder permits parallel or lengthy plate or grid leads tc augment the grid-plate capacitance. I am not insisting that Ci is the entire theoretical flea in the ointment, but practice supports the theory's implications. We have here also an explanation of the popularity and success of the tuned-grid tuned-plate transmitter circuit. The TGTP circuit is different from the Hartley circuit in one large respect, the larger reactive plate load. By formula, if R0 is high enough, variations of RP are swamped out and Ci variations cannot occur harmfully. Accordingly, the TGTP circuit should be stable without "Hi-C" complications. Don't try to reverse my earlier tricks and apply this to a tuned-plate receiver for there you vary your large RQ and acquire critical detuning like that which made the 1920 variometer tuner such quicksilver in the hand. • may, 1929 page 32 •