INTRODUCTION TO SONAR 



current only on the alternate half-cycles when 

 the plate and grids are positive with respect to 

 the cathode. The amount of current that flows 

 indicates the condition of the cathode-emitting 

 surface. On tube-testing equipment, the meter 

 scale usually is calibrated by dividing the total 

 pointer arc into three areas, which are labeled 

 GOOD, WEAK (or FAIR), and BAD. 

 • Diode tubes: The emission test for diode and 

 rectifier tubes and the diode part of multisection 

 tubes is similar to the emission test used for 

 multigrid tubes. The tube filamsnt or heater is 

 operated at the rated value, and an a-c voltage is 

 applied to the test circuit consisting of the diode, 

 a d-c milliammeter, and a variable resistor. A 

 tapped secondary is utilized in some circuits to 

 vary the amplitude of the test signal. The variable 

 resistor limits the tube current to a safe value. 

 The amount of current flowing through the resist- 

 ance and the meter depends on the electron 

 emission within the tube, and therefore indicates 

 the emission quality of the tube. 



Transconductance Test 



The term transconductance (also called mutual 

 conductance) expresses the effect of grid voltage 

 upon the plate current of a tube. By measuring 

 the transconductance of a tube, it is possible to 

 evaluate the tube's ability to amplify a-c signals 

 much more accurately than by measuring its 

 cathode emission, because this test more closely 

 approximates actual circuit conditions. Transcon- 

 ductance is expressed mathematically as the 

 ratio of a change in plate current to a smiTll change 

 in control grid voltage, with all other electrode 

 voltages held constant. Transconductance is 

 measured in units of conductance called mi- 

 cromhos . 



A/NA/VAA > 



1.76 

 Figure 10-18. — Basic circuit used for gas test. 



If the amount of gas in the tube is appreciable, 

 the collisions between the numerous gas molecules 

 and the cathode-emitted electrons release many 

 secondarily emitted electrons, and the resulting 

 flow of grid current is high. The basic circuit 

 used for the gas test is shown in figure 10-18. 

 With switch S set to position 1, a certain value of 

 plate current is measured by the d-c milliam- 

 meter. If there is no gas (or a negligible amount) 

 present in the tube, tlir owing switch S to position 

 2 does not change the plate current reading. If 

 gas is present, current flows through the large 

 value grid resistor, causing a voltage drop to 

 develop with the polarity as shown. The net 

 effect is to reduce the negative bias voltage on 

 the grid of the tube, resulting in an increase in 

 plate current. Sm.all plate current increases are 

 normal; large increases indicate excessive gas. 

 In some circuits a neon light is substituted for 

 the milliammeter and glows to show the presence 

 of excessive gas. 



Gas Test 



In all electron tubes, except some types of 

 rectifier and regulator tubes, the presence of any 

 appreciable amount of gas is extremely unde- 

 sirable. When gas is present, the electrons 

 emmited by the cathode collide with the molecules 

 of gas. As a result of these collisions, electrons 

 are dislodged (secondarily emitted) from the gas 

 molecules, and positive gas ions are formed. 

 Because the control grid is negatively biased, 

 the positive gas ions are attracted to it, and they 

 absorb electrons from the grid circuit in order 

 to revert to a more stable condition (not ionized). 



Short Circuit and Noise Tests 



The test for short-circuited elements must be 

 applied to a tube of doubtful quality before any 

 other tests are made. This procedure protects 

 the meter (or any other indicator) from damage. 

 Also, it follows logically, if a tube under test has 

 elements that ai'e short-circuited, there is no 

 further need to apply additional tests to that tube. 

 Short circuit tests usually are sensitive enough 

 to indicate leakage resistance less than about 1/4 

 megohm. The proper heater voltage is applied in 

 order to detect any tube elements that might short 



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