Radio Broadcast (Nov 1926-Apr 1927)

194 RADIO BROADCAST ADVERTISER Now you can build a really professional radio set. Mm WmfH kkiM \ TO build a really professional looking and efficiently operating radio set here are two new and important construction units. The Truphonic Power Amplifier, more fully described on another page of this issue, provides by far the finest type of audio amplification so far developed. For the set builder the Truphonic may be had in a Catacomb Assembly which gives you a complete unit containing the following: Complete Truphonic audio amplifying system including an output unit to protect the speaker from burning out and demagnetization, sockets with attached leads for the tuning and detector end of the set. The illustration shows how neatly this Catacomb Assembly houses all of these elements and how compactly it fits behind the tuning control. No holes to drill, no apparatus to mount. Short, direct leads with a minimum of soldered connections. This unit may be arranged in a hundred different ways to match all the requirements of every circuit and set design. A six foot battery cable is included, in which provision is made for the extra B batteries and C batteries for the use of power tubes. The Truphonic Assembly is provided in two models, one for 6 tubes, $20., and one for 7 tubes, $22. Localized a Control ^ Tuning Unit Trade Mark Reg. U. S. Pat. Off. The Na-Ald Localized Control Tuning Unit (Quadruple model shown) is a boon to the set builder — a great advance in multiple condenser construction. It can be used with any form of radio frequency coils, and gives you simple control under the fingertips of one hand, enabling you to tune all the condensers at once, or to tune each one separately and distinctly. These advanced Na-Ald Localized Control Tuning Units are provided in several models (all are of .000375 capacity unless otherwise indicated). Double #8., Double (.0005) #10., Triple $\o., Quadruple $15., Double with tickler control $10. With each unit is included the handsome panel plate shown above. Your dealer has these Na-Ald advanced construction units, or can get them for you. ALDEN MANUFACTURING CO. Dept. B-20 Springfield, Mass. Radio Broadcast Laboratory Information Sheet December, 1926 C Battery Detector FORM OF PLATE CURRENT TN THIS Laboratory Sheet we are going to discuss some points regarding the operation of a C battery type detector. We are going to consider, in particular, the form of the plate current of this detector tube. When no signals are being received, the plate current is constant and depends on the adjustment of the C battery. For best operation of a detector of this type, about four volts of C battery should be used on the 201-A when 45 volts are used on the plate. When a signal is received, the plate current varies and is then made up of two compon ents; one of these components is the pure d.c. current that flows in the plate circuit when no signals are being received and the other component is an alternating current which is produced by the audio frequency modulation in the carrier-waves that are being received. Although the detector is a rectifier, the current in the plate circuit is not in the form of a pulsating current as might be obtained from such a unit as a B line supply device, which is also a rectifier. When the signal is being received, the voltage is impressed across the input on the accompanying diagram. This voltage causes the grid to become alternatelyimore positive and then more negative than the voltage due to the C battery. However, the C battery voltage is such that a greater change of plate current takes place when the grid becomes more positive than it does when the grid becomes more negative; therefore, the current variations in the plate circuit increase more than they decrease and the result is that the average current in the plate circuit is higher than when no signal is being received. These current variations in the plate circuit can be detected if they are permitted to pass through a telephone. Also, if a transformer primary is placed in the plate circuit, the current variations will produce a varying flux in the core and will cause corresponding voltages in the transformer secondary, and these, in turn, can be impressed on a further tube and the signal amplified. Radio Broadcast Laboratory Information Sheet Tuning the Antenna Circuit December, 1926 POSSIBLE METHODS TO USE TN PRACTICALLY all receivers described to-day no tuning is used in the antenna circuit. Generally the antenna circuit is of fairly high resistance and therefore rather broad in tuning and, consequently, it is seldom worth while to accurately tune the circuit. However, some increase in signal strength can be obtained by approximately tuning the circuit to resonance. This tuning can be accomplished in several ways. The simplest method is to use a tapped inductance as shown in sketch A. The taps should be so designed that the antenna circuit is tuned to approximately 1500 kilocycles when the fewest numbers of turns are included in the circuit, and resonant at about 500 kilocycles when the total number of turns are in the circuit. The antenna circuit may also be tuned with a variometer, as shown in B. The variometer must be capable of being varied in inductance sufficiently to cover the broadcast band of frequencies. The disadvantage of tuning the antenna circuit is that it adds another control when the trend in design is toward the elimination of controls. Evidently, then, the solutionis to design an antenna tuning device that can be automatically controlled, perhaps by attaching some device to the shaft of the variable condenser, such as is done in the "Equamatic" system, to vary the coupling of coils between tubes in an r.f. amplifier. It should be understood that movements of the condenser across the secondary winding has a tuning effect on the antenna circuit if the coupling be tween the coils is close. As an example, the coil used in the antenna circuit of a Browning-Drake receiver has a tap at the center for connection to the antenna. Consequently, the antenna capacity (possibly reduced somewhat if a series condenser is used) is across half of the coil and has a decided effect on the tuning of the secondary circuit. Radio Broadcast Laboratory Information Sheet December, 1926 Radio Telegraph Transmission DIFFERENT TYPES OF WAVES TN TRANSMISSION work by telegraph there are several different types of waves used, these being illustrated in the accompanying drawing. The drawing A represents the type of wave radiated by a spark transmitter. This form of wave is known as a damped wave since it gradually decreases in amplitude. One of these wave trains is radiated each time that a spark takes place across the electrodes of the spark transmitter. Generally the spark frequency is about 500 per second, so that, if the transmitter was turned on, there would be 500 of these wave trains radiated every second. This type of transmitter is gradually being replaced by apparatus using vacuum tubes for the generation of the high frequency oscillations. The second form of radiated energy is illustrated in B, and is known as I. C. W., meaning Interrupted Continuous Wave. In this system, the energy is radiated in a series of wave trains similar to the radiations obtained from a spark transmitter, the difference being that the amplitude of the radiated wave is constant and docs not decrease as shown in A. Energy of this form could be obtained by supplying a transmitter from a plate battery in series with which there was arranged some form of interrupter which opened the circuit, say, 500 times per second. The third type of transmitted wave is known as C. W. or Continuous Wave and in this system. energy is radiated all the time that the key is pressed and it is not broken up as was shown in the two instances given, above. This form of transmission is a very common one and is used by the majority of the high-powered transmitter stations and in amateur work. if Examined and approved by Radio Broadcast -fa