Radio broadcast .. (1922-30)

.RADIO BROADCAST. to offset this difference between the actual and the indicated C voltages, and because of it the indicated voltage may be taken as the actual voltage. It is the voltage drop in the secondary of the transformer (true only in a.f. transformers) due to the cur- rent drawn by the meter. It will depend, of course, on the resistance of the second- ary, and the current drawn by the meter. As an instance, let us suppose a 112-type tube with 12 volts C bias and a trans- former secondary of 10,000 ohms. Using the 50-volt C scale, at 12 volts the meter will draw about 0.25 mA. This current passing through the secondary resistance of 10,000 ohms causes a drop of 2.5 volts across it. The indicated C bias, therefore, is 2.5 volts less than the actual bias, which just offsets the 2.5 volts in excess of the actual voltage due to the mid-tapped re- sistor, and the indicated voltage then equals the actual voltage. Another point to watch out for when taking C readings in audio stages is the error caused by the volume control in the audio stage. The method usually employed is to connect a 200,000-ohm potentiometer across the transformer secondary and to tie the grid of the tube to the movable arm of the potentiometer. The meter current then must pass through the parallel path formed by the resistance between the center con- tact and one side of the potentiometer, and the resistance between the center and the other side of the potentiometer. In the case of a 200,000-ohm resistor, if the vol- ume control happens to be half way on, the resistance in series with the meter would be 100,000 ohms, and the indicated C bias would be only one third of the actual bias. The remedy obviously is to turn the volume control all the way on. Plate circuit continuity is indicated when either the " 100V " or " 500V " button is depressed. If the voltage is not known to be less than 100 volts it is best to press the "500V" button first to prevent over- loading the meter. The voltage drop across transformer primaries, due both to the meter current and plate current of the tube must be taken into account to recon- cile the value of indicated voltage with the voltage at the source. This difference is inconsequential except when determining the voltage of B batteries by a reading at an a.f. socket, or when the meter load assumes a considerable proportion of the total load, as in the case of a plate voltage reading on a detector tube. In this case if the plate voltage were supplied through a series resistor from a high-voltage source, the current drawn by the meter—which might be equal to half of what the tube uses—would cause a 50 per cent, increase in the drop across the series resistor. The tube must be in the diagnoser socket when talking plate-voltage readings if the supply is from a power-supply unit. Plate current is read by pressing the 10-mA. or the 100-mA. button. It is al- ways best to press the 100-mA. button first, to be sure the current in not over 10 mA., and if it is not, to then press the 10-mA. button. The heater voltage of a 227-type tube is obtained by inserting the 227 test plug in the set socket, the tube in the diagnoser socket, and pressing the "227 heater volt- age" button. When taking a plate voltage reading on a 227, sufficient time must be allowed for the cathode to attain its work- ing temperature. From a consideration of the action of a vacuum tube as a voltage amplifier, it is clear that the ratio of plate current change to the grid voltage change causing it is the best figure of merit of the tube. This rela- tion is termed mutual conductance and is expressed in micromhos. Between two tubes of the same type, the one having the higher value of mutual conductance is the one better suited as a voltage amplifier. To measure the mutual conductance of a tube, with the diagnoser, divide the indi- cated value of grid voltage into the increase of value in plate current in milliamperes, when the ' grid-test" button is pressed and multiply this figure by 1000. The re- sult is the mutual conductance in microm- hos. The table accompanying this article' gives the average values of the change in plate current at the commonly used plate and grid voltages for most of the tubes encountered at present, together with their mutual conductance at these particular values of plate and grid voltage. Tubes of the same make may vary from 10 to 20 per cent, above or below the values given in this table. It should be remembered, however, that it is the change in plate current with change in grid voltage that is important. Sometimes two tubes of the same type will be encountered, one with a plate current swing from 1 mA. to 6 mA. amperes and the other with a swing from 5 mA. to 10mA. As voltage amplifiers one is as good as the other. The mutual con- ductance is the same for each. However, it would be preferable to use the one with the lower plate current in an audio stage since the audio transformer core would be worked at a lower flux density resulting in a higher value of permeability and hence increased primary impedance. The higher the primary impedance, the more uniform the voltage amplification. A HIGH-RESISTANCE VOLTMETER SUBSTITUTE By FREDERIC B. FULLER IT is WELL KNOWN that a low-resistance voltmeter cannot be used alone to mea- sure the e.m.f. across a device which has considerable internal resistance. How- ever, a milliammeter plus such a voltmeter can be used at considerable saving in cost over a high-resistance voltmeter. The theory may seem a little compli- cated, but the method about to be described is simple and quick. There is a definite, though unknown, amount of resistance between the two points whose difference in potential is desired. Let this resistance be R, let the current through it be I, and the true voltage across it be E. Then by Ohm's Law R=E/I expressed in ohms, volts, and amperes. Of these three unknown quantities, the current is de- termined most easily, by inserting a milliammeter in series with the load as shown in Fig. 1 (A) and noting its reading. Now connect the low-resistance voltmeter across both the milliammeter and the load, as shown in Fig. 1 (B). This will im- mediately cause a change in the reading of the milliammeter, as explained above. Let this new current reading be. I' and let the reading of the voltmeter be E'. •vpt Here again, by Ohm's Law, R = -p (the resistance of the milliammeter is prac- f? tically zero). Then, if we have R = i- 1 F F' T we know that r = T7 whence E = E' X T-, In other words, the true voltage across the mA. (A) (B) Fig. 1 load (that is, the voltage that will be there after the voltmeter has been removed) is the voltage indicated on the voltmeter multiplied by the ratio of the ammeter reading before the voltmeter was applied to the ammeter reading after the voltmeter was applied. One simple example will be given: I = First ammeter reading = 0.030 amperes I' = Second ammeter reading = 0.024 am- peres E' = Voltmeter reading = 36 volts E = True voltage =^j£. X 36 = 45 volts U*v24 This method is absolutely accurate, re- gardless of how low the resistance of the voltmeter, provided, of course, that both ammeter and voltmeter are themselves calibrated accurately. Incidentally, the resistance, R, of the TCV load is learned by this method, for R = -^- which in the above example, is 1500 ohms. 36 0.024 Fig. 2 Measuring Plate Current In measuring the voltage across a tube it makesa difference which side of the ammeter the voltmeter is connected to. In Fig. 2 a voltage divider, "D", is shown, and two tubes, M and N. If it is desired to learn the plate-to-filament voltage of tube M, for example, the "load" is the plate-to- filament resistance of tube M. The ammeter should be connected close to tube M, not close to the voltage divider, and then the voltmeter attached across both the ammeter and the tube, as shown. There will be a slight change in the plate resist- ance of the tube due to the slight change in its plate potential when the voltmeter is applied. But this is so small that it may be neglected entirely in most calculations. 54 • • NOVEMBER 1929