Radio Broadcast - 1927, May - 61 Pages, 4.9 MB ... - VacuumTubeEra

MAY, 1927phone levels in TU's. The form shown in Fig.i, however, is intended primarily as an indicator.The input transformer shown may be an audioamplifying transformer, preferably with a highstep-upratio (1:10 or 1:5). The primary impedancemust be sufficiently high sothat, even at low frequencies, theinstrument will not affect circuits ofabout 500 ohms impedance, acrosswhich it is bridged. This means that at100 cycles the primary of the transformershould have an impedanceInputof20,000 ohms, which requires 40 henrysinductance, approximately. The secondaryof this step-up transformerfeeds a vacuum tube, which may be ofthe UX20I-A type, or its equivalent.The grid of this tube is biased negativelyto about 10 volts, so that theplate current, measured on a milliammeter,MA., of o-io range, is onlyabout 0.5 milliampere with 90 voltsplate potential. A filament voltmeteris provided to keep the voltage acrossthe filament constant.The theory of operationis as follows:Through the negative biasing ofthe grid, the tube is being worked sofar down on the curve that when thetransformer secondary contributes afurther negative potential to the gridInputthe plate current is only insignificantlyreduced. See the characteristic curveof Fig. 3. On the other hand, positive potentialscarry the current up on to the steep portionof the curve once more, resulting in a flickupward of the milliammeter pointer. The amplitudeof this movement indicates the magni-VOLUME INDICATORSshort of transmitter overloading, is 5.0. Then itis the control operator's business not to allowhis peaks on the volume indicator meter to hitmore than 5.0, and, even if he does not know theabsolute value of the r. m. s. audio voltage heFIG.STATIC CHARACTERISTICUX-201 -A90 V. PlateImfd.Reacta nee (Am plifyingTransformer with Primary& Secondary in series, additively )sufficiently on the second tube, which is thevolume indicator proper. Both in the one- andtwo-tube outfits, various combinations oftransformers, tubes, and indicating instrumentsmay be utilized. An ordinary amplifying transformerof i :3 or i .-5 ratio, a highmutube, and a galvanometer ofabout 1.5 milliamperes full scalereading, is a good single-tube combination.The voltages may be aboutthe same as with the high ratioinput transformer, ux-2oi-A tube, ando-io milliammeter described below.Mr. W. K. Aughenbaugh sends ina sketch of a volume indicatorusing two ux-i I2's, 45 volts plate, noGRIDVOLTSgrid bias, ordinary amplifying transformers,and a o-io d.c. voltmeter asan indicator. A swing from 2 to 6 onthe meter gives him the proper modulationon the WFBG transmitter, whichhe runs. This is not really an orthodoxvolume indicator, and it wouldbe improved with the negative biasfeature, but it serves the purpose. Sincethe instrument is used only as a leve'indicator, the proper limits of operationhaving been established by listeningchecks on quality and overloadobservations in the amplifiers andtransmitter, the operators at eachMA. D.C.0-10 =; .Milliamperes s90V.3FIG. 2tude of the alternating potential impressed onthe grid, and hence, divided by the transformervoltage ratio, the audio potential on the lineacross which the volume indicator has beenbridged. The device ismerely a special form ofrectifier. Fig. 4 is a table, in which columns i and2 are taken from the characteristic curve ofFig. 3, showing the variation of plate currentwith grid potential. Column 3 shows the plusrise in voltage calculated from the base point,which is the negative grid bias potential, 10volts in this case. Column 4is this figure dividedbythe transformer ratio, assumed as 10; thisgives us a rough measure of the audio voltageacross the input of the instrument. By tappingvarious potentials along the grid bias batteryone may calibrate a volume indicator, for comparativepurposes, in this way. The results shouldnot be taken as absolute values, because theresponse characteristic of the meter, the terminalconditions in the circuit being measured, andother factors, all affect the readings. However,these unknowns may be disregarded when onlyindications are required. For example, with atube of normal emission in the volume indicator,and proper filament and plate voltages, it mayhave been ascertained that, when the volumeindicator isbridged across the line leading tothe radio station, with given settings there,the maximum allowable deflection of the meter,