Radio mirror (Jan-Oct 1923)

RADIO DIGEST ILLUSTRATED 13 The By-Pass Condenser in the Reflex Circuit Part III — Reflex Discussion Gives Three New Hook-Ups By H. J. Marx THE key to reflex circuits lies in the proper use of by-pass condensers. For this reason the fan should thoroughly understand their functions and the reasons for their use. Alternating: current when passing through a coil or winding of wire encounters a resistance or choking effect, due to the induction from the magnetic field which is created. As the magnetic field is intensified by the introduction of iron cores, this resistance or choking effect is decidedly increased. Direct current however is unaffected, the magnetic field remains constant and unbroken, therefore no induction takes place. Alternating current will therefore flow through the primary winding of a Radio frequency transformer! which usually has an air core, an iron dust core, or even in some cases a soft iron core of small diameter. This same current however Will not pass through the primary winding of an audio frequency transformer, due to the iron core and increased number of turns of very fine wire. Then again alternating current will pass on through a condenser whereas a direct current will not. Audio Primary Must be By-Fassed In reflex circuits the current comes through as an alternating current of high frequency. This current in passing through the plate circuits first goes through the primary of the Radio frequency transformer, where it encounters but little resistance to its passage. The next winding through which it must pass is the primary of an audio frequency transformer. As just explained the resistance is too great. In order to overcome thi», the primary of the audio frequency transformer is shunted with a fixed condenser through which the audio frequency current is enabled to pass instead of the transformer winding. Now to follow through what happens after the current has passed through the series of stages the first time. Whether a crystal or tube detector is used, the current has been rectified and is then a pulsating direct current in form, and of much lower frequency. Having been reflected back to the tubes all over again, it passes through the primary winding of the Radio frequency transformer and encounters very little resistance in passing through it. When it gets to the primary of the audio frequency transformer, it goes through the winding instead of the condenser, as was the case before rectification. Audio Transformer Secondary Effects The question may be asked why it is that the secondary windings of audio frequency transformers are not always shunted with by-pass condensers. It will be found that the natural capacity of such a winding is usually sufficient to permit the Radio frequency currents to pass through. There seems to be a wrong impression that this by-pass condenser should be variable and is added for tuning the primary circuits of the various stages to the proper wave lengths. The variable resistance sometimes added in the plate circuit, is used for stabilizing the various stages through which two different frequency currents are passing. Hooi-Up Figure 1 The Figure 1 three-tube reflex hook-up is very similar to the one published in the first article of this series. Transformer coupling is used throughout, however. Three Radio frequency and two audio frequency are required. A crystal detector is used for rectification of the Radio frequency current. A potentiometer is shunted across the A battery and is battery. This also acts as a by-pass for Radio frequency currents. The variable condenser in the primary circuit has a capacity of .001 mfd., while the one shunted across the secondary need not have more than .0005 mfd. All tubes have separate rheostat control. This circuit will be found selective and simple to operate. The plate voltage should be about 80 volts. Hoot-Up Figure 2 The same type of coupling, with, how the antenna is short. A .0005 mfd. variable condenser is used to tune the secondary circuit. The grid potential of all stages except the detector, is controlled by means of a 400-ohm potentiometer connected across the filament or six-volt battery. A .002 mfd. fixed condenser is connected in between the moving lever of the potentiometer and the negative terminal, acting as a by-pass for any stray Radio frequency currents. Fixed condensers of .002 mfd. capacity are shunted Figure 2 used for controlling the potential of the grid of the first tube. High variable resistance such as potentiometers (400 to 1,000 ohms or even more) are inserted in series in the plate circuits of the second and third tubes. A .002 mfd. fixed condenser is connected from the moving lever of the potentiometer to the negative side of the filament ever, a tube detector, is illustrated in Figure 2. This circuit is likewise three stages of Radio frequency, detector and two stages of audio frequency. A variocoupler, with two tap switches for both rough and fine adjustment, is used for the tuning. A .001 mfd. variable condenser is added in series in the antenna circuit although this may be omitted if GROUND JACK FOR. Plugging in !>LOOP AERIAL: O-A + O O-B + O Figure 3 I5IA.F. Z^AF across the secondary of the first audio frequency transformer, across both windings of the second audio frequency transformer, across the plate batteries, and also across the phone or output terminals. In spite of the apparent complex nature of the set, it presents no serious difficulties in tuning. The potentiometer is apt to be rather critical if its resistance isn't high enough. The secondary condenser is also apt to be critical unless a vernier is used. The plate voltage can be increased to as high as 100 in the amplifying stages but it is best to test out for the voltage giving the best results without distortion or mushing of signals. The filament rheostats will have decided effectiveness in selectivity for distant stations. In fact it would be advisable to use a vernier rheostat for the detector tube. The grid leak used in the detector grid circuit might be of the variable type, so that the best grid reaction adjustment can be obtained. This will vary for the different types of detector tubes that may be used. Hook-Up Figure 3 There is no reason why jacks should not be incorporated in reflex circuits. Although not called for in the Figure 3 illustration, filament control jacks may be substituted. If this is done, both jacks should be connected so that current is furnished to all the tubes when a plug is inserted in either jack. The last two jacks are for plugging in either the first or second stages of audio frequency. The first jack (double-circuit type) permits the use of a loop aerial, automatically cutting out the variocoupler which would then be unnecessary. The variable condenser in the primary circuit should have a capacity of .001 mfd. The secondary variable condenser is of the vernier type, having .0005 mfd. capacity. It will be noticed that only the first stage of Radio frequency uses the potentiometer for grid control. The usual .002 mfd. condensers are used for the by-pass of all Radio frequency currents throughout the circuit. When either jack is in use, (Continued on page 14) .. M