Radio Broadcast (May 1928-Apr 1929)

A Few Radio Questions Answered rHY is the volume control placed in the r.f. amplifier ? It need not be; it may be connected in the antenna-ground circuit, in the r.f. amplifier, in the detector, or in the a.f. amplifier. It preferably should be connected in a circuit ahead of the detector, so that the signals may always be kept below the point where detector overloading begins. This is particularly important when a leak-condenser detector is used — because it has a low overloading limit. It is our guess that 99 per cent, of the sets now in use have this kind of detector. If the volume control is placed on the audio amplifier, that is, after the detector, distortion is liable to result on strong signals, even though the volume from the loud speaker is low. This is due to too strong signals being placed on the detector input. T HAVE a lyi-type power tube. Is there any ad-*■ vantage in using a iji A-type tube ? The only advantage is in the improved economics of your radio system. The new tube will not deliver more volume, nor will it last longer, nor will it give better quality. It consumes half the A-battery current; hence, it is about twice as efficient, if you chose to call the efficiency of the tube the amount of audio-frequency output it will deliver per watt of power used up to heat the filament. T DROPPED my audio transformer and now the ■*■ quality seems "sour." What is wrong ? The chances are that your audio transformer had a high-permeability core made of one of the new alloys of iron, nickel, etc. It is a fact that the greatest care in manufacture is necessary not to lower the permeability of the core material by severe mechanical shocks. When the permeability is high, a relatively small core and relatively small amount of wire will produce a high-inductance winding. When a shock lowers the permeability, the inductance goes down, and the low frequencies fall out. It is a standard physics class experiment to drop iron rods on a hard floor in such a direction that at the moment of contact the molecules can be oriented properly with respect to the earth's magnetic field. After a sufficient number of shocks, the iron bar will be found to be permanently magnetized. It is possible that your audio transformer has become permanently magnetized— which would have the same effect as sending too much d.c. current through it. It "saturates" easily. "LJOIV can I tell if my tubes need replacing ? -*• According to certain publicity writers a new tube should be placed in each socket of a set least once a year — but don't you believe it. Just because you bought a tube a year ago to-day, is — Band ; — Passed : 45 50 55 60 KILOCYCLES FIG. 2 65 NEARLY everyone wants to know the answer to some one question about radio. Many want to know the answer to specific questions, such as " What is wrong with my set, the tubes don't light?" Others want to know the answers to general questions: answers which should be obvious but are not. In this page the Laboratory Staff has attemped to answer a few of the questions that are asked it many, many times, and in this paragraph expresses the hope that readers who have other similar questions will not hesitate to send them in. — The Editor no reason why you should throw it in the wastebasket and invest in a new one provided, of course, a test proves it to be a good tube. Tubes do not run down in just that manner. The Laboratory has records of a number of Sylvania tubes which ran on a life test for 1500 hours without any change in their constants — except a minor improvement in some of the tubes — and were then taken off and used around the Laboratory for months afterwards. Some tubes last 1000 hours, others become anaemic at the end of a few hundred hours — for no reason that anyone can state. The tubes may have come from the same plant and exactly the same run — but something in their make-up gave them a short life. If your set seems to have slowly given up its sensitiveness to weak signals, if it no longer gives out low notes, and if you can't get dx on a crisp cold December night when your next door neighbor gets pwx, you'd better have your tubes tested. When the quality seems bad, and the amplifier overloads easily, look to the 1 7 1 -type power tube which is probably being run on a.c, and whose filament has been overloaded so that it no longer has sufficient electrons to handle low or loud notes. Any reputable dealer can test the tubes. WH ERE should a filament rheostat he placed, in the positive or negative lead ? Should the minus B wire be connected to minus or plus A wire? Look at Fig. 1 . Here th e resistor, which may be variable or not, is connected in the negative filament lead. The bottom end of the coil attached to the grid is connected on the battery side of this resistance. There is a voltage drop across this resistance, which makes the end 166 nearer the battery one volt more negative than the end near the tube. Since all tube voltages are measured with respect to the negative end of the filament, this voltage drop is applied to the grid. We can say, then, that the grid is one volt negative, meaning that it is actually one volt more negative than the negative end of the filament (Eg= — 1). If the coil were attached to the filament end of the resistor, the grid would be at the same potential as the filament, and would be at zero bias (Eg = o). It makes absolutely no difference in this case if the resistor is in the plus or minus lead. When minus B is attached to minus A, the plate potential is the voltage of the B battery. When minus B is attached to plus A, the plate potential is the voltage of the B battery plus the voltage of the A battery, because plus A is above the voltage of minus A — which is connected to the negative filament lead — by the voltage of the A battery. Thus, if the B battery is 45 volts, and the A battery is 6 volts, and minus B is attached to plus A, the plate potential is 45 plus 6 or 51 volts. If the minus B is attached to minus A — and we prefer such a connection — the plate potential is the same as the B-battery voltage, namely, 45 volts. For years telephone circuits have connected minus B to plus A, but we don't see any good reason for it except tradition. /^AN a high-mu tube such as the 240 be used as an r.f. amplifier ? Yes, and it will make a good one too. The trouble is that the amplification will go up a bit faster than the selectivity, so that the circuit seems to tune broadly. If a transformer is used to couple a high-mu tube to another amplifier tube or to a detector, the turns ratio must be greater when using a high-mu tube than when using a 201 A-type tube. An explanation of this turns ratio business may be found in "Strays from the Laboratory " September and December. THAT is a band-pass amplifier ? W Strictly speaking it is an amplifier that admits, amplifies, and transmits only a certain band of frequencies, say from 50 to 60 kc, and refuses all other frequencies. Strictly speaking again, there is no such thing. All band-pass amplifiers admit a certain amount of currents of other frequencies, but this amount can be made quite small. A true band-pass amplifier characteristic would look like Fig. 2, and the amplifier would consist of a great many stages of filters, each composed of inductance and capacities. A band-pass amplifier which does not have a sharp "cut-off" would look like Fig. 3, and a sharper one would look like Fig. 4. Both admit frequencies on either side of the desired 50-60 kc. band but in smaller amounts. Theoretically it is possible to build a band-pass amplifier with a flat top and steep sides. Whether or not an amplifier has such a characteristic in practice has not been determined in the Laboratory. 40 45 50 55 60 65 KILOCYCLES FIG. 3 45 50 55 60 KILOCYCLES FIG. 4