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Philips DEALING with TECHNICAL PROBLEMS VOL. 5 N~. 3 . Philips DEALING WITH TECHNICAL PROBLEMS. RELATING TO THE PRODUCTS, PROCESSES AND. INVESTIGATIONS OF N.V. PHILIPS' GLOEILAMPENFABRIEKEN EDITED BY THE RESEARCH LABORATORY OF N.V. PHILIPS' GLOEILAMPENFABRIEKEN, EINDHOVEN, HOLLAND TESTING A~PLIFIER OUTPUT VALVES BY MEANS OF THE 'CAT~ODE RAY TUBE hy A. J. HEINS van der VEN. 621.317.755: 621.396.645 I n testing amplifier output valves, the most important data are contained in the In' v.. diagram if one knows over which part of the diagram the values of voltage and current prevailing during operatien range, i;e. if the position of the load line is known. The la- Vn 'diagrams as well as the load lines can very easily b~ obtained with the help of a 'cathode ray tube. The necessary apparatus is described in this article. A number of auxiliary ar- rangernents are also studied, by which the axes and the necessary calibration lines in the diagram can be traced on the fluorescent screen, and which make it possible to. cause the diagrams of two .output valves which are to be compared to appear simultancously on the screen. In order to obtain the load line in the correct place in the diagram, use must he made of dir'ect current push-pull amplifiers for the deflection voltages of the cathode ray tube. The position of the load line upon inductive loading is discussed and explained by a number of examples. In conclusion one,npplication of the instaflation in the develop- ment of output pentodes is dealt with. If one wishes to characterize briefly the perfor- quantrties are to some degree dependent _on the mance of an output valve of !tn amplifier or radio loading impedance which is included in the anode set, it is enough to give the sensitivity, the distor- circuit, it is first necessary to find out how the loa'd- tion as a function of the power output, and in some ing of the ~alve is e~pl'esse~ in the Ia- V~ curve.' , cases the maximum power which can he delivered mA without grid current flowing.. ~or a more careful 120 examination, however, particularly for the dis- 100 19.0 covery of caus,es of deviations or errors, such for example as differences in distortion in different valves, one m~st in general have reference to the -4V Ia- Va diagram, which. forms the actual basis fo~' -6V the judgement of the properties of an output -8V valve. This diagram (fig. 1) gives the variation of ---tOY -t2V 100 the anode current Ia as a function of the anode 200 300 -Va 400 SOOV voltage Va with the negative control grid voltage 3'476:11' Vg as parameter 1). From the series of curves the Fig. 1. The la-V.. diagram of the output valve EL3' W is the operating point; in this valve it ordinarily lies at v., = 250 primary quantities of the valve: slope, internal volts, 1., = 36 mA, v:~= -6 volts. The screen grid voltage resistance and amplification factor, may be read is permanently fixed at 250 volts. The slope S is given by the vertical distance between two successive curves, the ampli- off immediately for every operating point, as is fication factor g by the corresponding horizontal distance, explained in the text under the figure. A connection the internal resistance by the slope of the la- VIZ curve at the operating point. AB is the load line for a pure resistance of may, however, also be found between the shape of 70PO ohms. the curve and' the above-mentioned quantities such as distortion and power output. Since these If the valve is loaded with a resistance R which is connected to the anode circuit through a trans- 1) The screen grid voltage, .which forms a second parameter in the 'case of pentodes, is kept constant for the whole· former in order to have no D.e. voltage drop in R diagram. (fig. 2), an alternating current IR will begin to '62 PHILIPS TECHNICAL "REVIEW Vol. 5, No. 3 flow in R when an A.C. voltage is applied to the by the load ·lines. A knowledge of this region makes grid of the valve. The A.C. voltage on the loading it possible to estimate the importance for the reo' resistance is then VR = R . I l'/' The total anode production, of any irregularities occurring in the voltage Va and anode current la of the valve are la- Va diagram. given by the sum of VR or IR and the values of In the following a method will be described by the anode D.C."voltage or current indicated by the which not only the Ia-Va diagram, but also the eperating point. The-relation between I a and Va of load line can very easily be obtained. The principle the loaded valve, the socalled load line, is therefore of this method, which makes use, of the cathode in this case represented by a straight line through ray tube, has already been outlined in this period- the operating point (AB in fig. I), with the slope icalê), at least as far as the recording of I.a-Va tan a = R with respect to the la axis. diagrams is conéerned. We' shall here go more deeply into the practical execution of the method and also give several examples of its application. Instaflation for the 'recording of la- Va diagrams. (} An la- Va cl,taracteristic can be registered on the screen of a cathode ray tube in the following way . -s- A given D.C. voltage is applied to the control grid Vg2 Va 3'4764 of the output valve to be investigated, while the anode voltage is made to vary periodically 'from Fig. 2. A loading resistance R is included in the anode circuit . of the output valve via a tr".nsformer 1:,1. zero to a maximum value. This anode voltage is applied, via a potentiometer and an amplifier, to If one considers the grid voltage to be varying, the plates for horizontal d~flection of the fluorescent the values of la and Va at any moment are always spot. The voltage on a small measuring resistance, determined by the point of intersection of the load through which the anode current flows, causes the line with the la- Va curve corresponding to the value vertical deflection, which is therefore proportional ·of the grid voltage ~t that moment. If,for instancè, to the anode current at every moment. the operating point lies On the curve for Vg = -6 In order to trace different la- Va curves on the volts .and the grid excitation voltage is sinusoidal screen successively, the grid bias must he given with ~. peak value of 5 volts, then the' points of different values successively for short times. A inter~ecti?n ofthe line Al! with thecurves Vg.=-1 voltage which variès in steps must therefore be ·volt and Vg - -ll volts give the extreme values applied to the grid, and in order to obtain a lasting of la and Va. The distance AC is therefore twice image on, the screen of the cathode ray tube, this the amplitude of the altemating current occurring, voltage varying in steps mu~t be run through the distance BC twice the amplitude of the A.e. completely several times per second. voltage. The power output to the loading resistance The stepwise varying voltage is obtained by at the grid excitation voltage in question is thu's means of a rotating switch SI which passes over given by 1/4 of the area of the triangle ABC. : 28 contacts (fig. 3). Sixteen of these are connected Insofar as the successive la- Va curves cut off to the taps. of a potentiometer which correspond equal segments of the line AB, a proportional to the 15 steps of the. desired voltage series, see change in anode voltage and' current corresponds fig. 4. On the following 6 contacts (17 to 22) there to any change in the grid voltage. There is therefore is a high negative voltage, so that the valve to be - no distortion. With large amplitudes along the load examined passes no anode current at all during .line, however, the segments cut off on AB become this time. By this means, and by the great rapidity gradually smaller (fig. I). The variation öf .anode at which the switch rotates (1000 to 1500 r.p.m.] voltage and current with a sinusoidal grid excitation it is possible to record the whole la- Va diagram 'voltage is then no longer sinusoidal, but exhibits without the valve becoming overloaded .. a flattening, The distortion occurring could he For the sake of orientation in the diagram, the calculated by careful measurement of the segments two axes and the corresponding scale are necessary cut off on the load line. in addition to the la- Va curves. The remaining ,If the loading impedance is not a pure resistance, . ·the ioad Iine is not straight In the la- Va diagram. 2) H. van Su ch t el en; Applications of cathode ray tubes, Depending on the grid excitation voltage a larger or Philips techno Rev. ,3, 339, 1939. See also Philips techno Rev. 4, 56, 1939, where à related problem had to he solved smaller portion of the whole diagram is the~ 'c?ve~ed in a different way •. MAÎfCH 194,0 TESTING OUTPUT VÄI..YES 63 Fîg, '3. The anode voltage of the output valve L1 to be investigated is applied to the plates for horizontal deflection of the .cathode ray tube K via the amplifier VI; the voltage over the measuring resistance Rl\f, through which the anode current flows" is applied via the amplifier V; to the plates for vertical deflection.
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