Radio Broadcast (May 1927-Apr 1928)

JANUARY, 1928 A VACUUM-TUBE VOLTMETER 223 made only after due deliberation of what will happen once the proposed adjustment is carried out. After the experimenter becomes familiar with his apparatus, he will be able to proceed rapidly from calibration to operation, and to have such a feeling for his instrument that he can predict what each change in voltage, etc., will produce. CALIBRATION THE first job after the assembly of the vacuum-tube voltmeter is the calibration process. This is a fairly simple operation and need not be baulked at by the experimenter with average experience in handling measuring instruments. Before starting, bear in mind that too much current in the plate circuit of the tube will ruin the meter, so every step should be proceeded with gradually and with the utmost care. But before the actual calibration is started, we must decide upon the voltage of the C and B batteries in the voltmeter circuit itself (the A voltage is, of course, that recommended by the manufacturer of the tube used). We are assuming in these experiments that a 199 type tube will be used. The microammeter should not be connected in circuit until the potentiometer switch arm is thrown as far as possible towards the negative end or, preferably, a small piece of paper should be slipped under the movable contact to insulate it from the wire beneath. This prevents any of the voltage across the potentiometer, due to the filament battery, flowing through the microammeter until needed. Since this current is backwards with respect to the meter, it may damage it until some plate current flows. Now adjust the filament rheostat until the voltage is normal. Before proceeding further, we will test the circuit to see that everything is satisfactory (the actual calibration has not yet begun). We shortcircuit the input to the tube by connecting together points X and Y, Fig. 2. Assuming that the microammeter has been connected in circuit and that the B and C batteries are of satisfactory value, a current will flow in the plate circuit and will be indicated by a deflection of the meter needle. The value of this current is dependent upon the voltages of the B and C batteries. Thus it will be seen that the value of these batteries is all important for if they are of such value as to give a larger plate voltage than the microammeter can indicate, it will be at this point that the abrupt and disheartening termination of experiments will occur. Do not, therefore, play about with different values of plate and grid voltages unless you know just what you are doing. With a 199 type tube, we recommend that the B voltage be 45 and the C voltage be 9 to start with. Under these conditions a reading of 6 microamperes was obtained in experiments in the Laboratory. Fig. 3 shows some typical curves which were obtained in the Laboratory. They were obtained with a 199 type tube using a grid bias (Eg) of 5.8 volts and a filament voltage (Ef) of 3. Of the several curves shown, only that marked A. C. = 0 interests us at the moment. The term A.C.=0 indicates that there is no alternating-current input to the vacuum-tube voltmeter (in other words, X and Y are shorted. See Fig. 2). We note in the curve A.C. = 0 how rapidly the plate current increases with small increases in plate voltage. At 40 volts plate potential (Ep), for example the plate current (Ip) is 1 10 microamperes ((J Amps.). If with, say, forty volts plate potential, we increased the grid voltage, the plate current would decrease. The other curves in Fig. 3, incidentally, show the effect of placing an a.c. input across the terminals X and Y; in other words, they represent microammeter RAD L 10 BROAI ABORATO (CAST RY I 1 i « I 8 1 // '// M // 1 I 1 1/ v/ 1S/SS 1 2i / t t r 1! Eg«5 9 8;EF>3 pUSg^J 1 1 1 L 0 1 2 3 4 5 AC. VOLTS FIG. 4 Here are some typical calibration curves. The deflections of the microammeter needle caused by various a.c. input voltages are plotted, together with the effect of increasing the plate voltage (Ep) for a given value of C bias (Eg) deflections due to the sum of the normal plate current and the current caused by input a.c. voltage. We have digressed a little to go into a brief explanation of the curves in Fig. 3. We had previously arrived at that point where it was recommended that the experimenter use 45 volts B battery and 9 volts C battery. Under these conditions, it was stated, a plate current of 6 microamperes was obtained in the Laboratory. Do not expect, however, to get a similar reading. Every tube will vary. It is quite possible that you will get no reading whatsoever. Perhaps the reading will be higher than 6 microamperes. Should there be no reading on the microammeter, slightly reduce the C voltage until a reading of a few microamperes is apparent. It is quite possible that a C battery quite a little smaller than a 9-volt one will ultimately be used, but the large value is recommended for safety's sake. The larger the C battery the smaller the plate current and, therefore, the greater the margin of safety. Suppose that we have adjusted our batteries and have obtained a reading of io microamperes. We are satisfied that everything is working as it should, and take the next step, which is to balance out this 10-microampere reading. If we do not balance it out it will always be present when we are measuring a.c. voltages applied across X and Y and, therefore, the microammeter will not be used to its full advantage. That is to say, if the meter is always indicating 10 microamperes with no input across X and Y, its range will be reduced when an a.c. voltage is applied. True indeed, with a 200-microampere meter this 10 microamperes only represents one fiftieth of the whole scale, but possibly the combination of B and C voltages used will give a greater plate reading, perhaps 50 microamperes (such a high value would not be probable with a 199 tube and 45 volts B and 9 volts C), which certainly should be balanced out. The balancing out is accomplished by removing the piece of paper which we previously slipped underneath the potentiometer arm and adjusting the latter — slowly — until the needle on the meter reads zero. Some experimenters prefer their meters to indicate a few microamperes current when there is no a.c. input across X and Y so that any possible back deflection of the needle will not harm the meter. The actual calibration of the instrument is our next job, and it is not an extremely difficult A VERY COMPLETE VACUUM-TUB E VOLTMETER The complete instrument may be dressed up to look like this if you do not care for the breadboard layout shown in the photograph on page 22 1 . Voltmeters to read the B and C potentials are included