HANDBOOK Mixer and Converter Tubes 79 stituted for the plate of a triode. p5g denotes The Effect of The current equations show how the ratio of a change in grid voltage to a Grid Current the total cathode current in change in screen voltage, each of which will triodes, tetrodes, and pentodes produce the same change in screen current. is a function of the potentials applied to the Expressed as an equation: various electrodes. If only one electrode is positive with respect to the cathode (such as AE,, would be the case in a triode acting as a Ps: 1 sg = constant, A = small class A amplifier) all the cathode current goes AE, increment to the plate. But when both screen and plate are positive in a tetrode or pentode, the cath- The grid- screen mu factor is important in ode current divides between the two elements. determining the operating bias of a tetrode Hence the screen current is taken from the or pentode tube. The relationship between con- total cathode current, while the balance goes trol -grid potential and screen potential deter- to the plate. Further, if the control grid in a mines the plate current of the tube as well as tetrode or pentode is operated at a positive the screen current since the plate current is potential the total cathode current is divided essentially independent of the plate voltage between all three elements which have a posi- in tubes of this type. In other words, when tive potential. In a tube which is receiving a the tube is operated at cutoff bias as deter- large excitation voltage, it may be said that mined by the screen voltage and the grid - the control grid robs electrons from the output screen mu factor (determined in the same way electrode during the period that the grid is as with a triode, by dividing the operating positive, making it always necessary to limit voltage by the mu factor) the plate current the peak -positive excursion of the control will be substantially at cutoff, as will be the grid. screen current. The grid- screen mu factor is numerically equal to the amplification factor Coefficients of In general it may be stated of the same tetrode or pentode tube when Tetrads: and that the amplification factor it is triode connected. Pentodes of tetrode and pentode tubes is a coefficient which is not Current Flow The following equation is the of much use to the designer. In fact the ampli- in Tetrodes expression for total cathode cur - fication factor is seldom given on the design ond Pentodes rent in a triode tube. The ex- data sheets of such tubes. Its value is usually pression for the total cathode very high, due to the relatively high plate current of a tetrode and a pentode tube is the resistance of such tubes, but bears little same, except that the screen -grid voltage and relationship to the stage gain which actually the grid- screen p- factor are used in place of will be obtained with such tubes. the plate voltage and p of the triode. On the other hand, the grid -plate transcon- ductance is the most important coefficient of / E 5/2 pentode and tetrode tubes. Gain per stage can Cathode current = K 1 Eg + sg be computed directly when the Gm is known. Psg The grid -plate transconductance of a tetrode Cathode current, of course, is the sum of the or pentode tube can be calculated through use screen and plate current, plus control grid cur- of the *expression: rent in the event that the control grid is posi- Al tive with respect to the cathode. It will be Gm = -AEg noted that total cathode current is independent of plate voltage in a tetrode or pentode. Also, with Esg and ED constant. in the usual tetrode or pentode the plate cur- The plate resistance of such tubes is of rent is substantially independent of plate less importance than in the case of triodes, voltage over the usual operating range- which though it is often of value in determining the means simply that the effective plate resist- amount of damping a tube will exert upon the ance of such tubes is relatively high. How- impedance in its plate circuit. Plate resist- ever, when the plate voltage falls below the ance is calculated from: normal operating range, the plate current falls sharply, while the screen current rises to AE, RD such a value that the total cathode current remains substantially constant. Hence, the D screen grid in a tetrode or pentode will almost with Eg and Esg constant. invariably be damaged by excessive dissipa- tion if the plate voltage is removed while the 4 -5 Mixer and Converter Tubes screen voltage is still being applied from a low -impedance source. The superheterodyne receiver always in- www.americanradiohistory.com.