Basic Theory of Mutual Conductance Test

circuit gives an indication of the overall merit

of amplifier tubes under simulated operating

conditions, by measuring the mutual conduct-

ance (G m) of the tube under test. When push-

button 3 -- MUT. COND. is depressed, the

mutual conductance of the tube is indicated on

meter M101 in terms of arbitrary units from

0 to 120. To convert the numerical value of the

meter reading to mutual conductance in mi-

cromhos, refer to paragraph 20, TM 11-6625-

274-12.

Figure 4. Rectifier diagram.

f. Gas Test Circuit. This circuit is used to

at the rate of conduction of the tube because of

check for the presence of excessive amounts of

the inertia of the meter movement.

gas in vacuum-type tubes. Excessive gas is in-

b. If the amplifier tube to be tested is sub-

dicated by a change in the position of the meter

stituted for the fixed load resistance and a

pointer due to the shift in the operating points

fixed bias voltage E is applied to the tube (fig.

of the tubes because of gas current in the grid-

5), the meter will still indicate zero because

to-cathode circuit. The gas test is performed by

the amplifier tube, under steady-state condi-

depressing first pushbutton 4 -- GAS 1, then

tions, acts like a fixed resistance.

pushbutton 5 -- GAS 2.

c. If, in addition to the bias voltage, an ac

potential is applied between the control grid

5. Basic Theory of Mutual Conductance Test

and the cathode of the tube under test, the cir-

To better understand the function and opera-

cuit becomes equivalent to the one used for

ion of the mutual conductance test circuit used

mutual conductance tests in the TV-7(*)/U.

in this test set, a brief review of the basic

When the ac potential causes the bias voltage

principles used follows.

between the control grid and the cathode to be-

a. The two secondary windings of the trans-

come less negative, plate current through the

former (fig. 4), which are energized from a

tube will increase. Since the plate-cathode re-

60-cycle power source, supply plate voltage to

sistance has decreased, more current will flow

the full-wave rectifier tube. The inner end of

t h r o u g h resistor R M , and the deflecting force

each secondary winding connects to one side of

on the pointer of the meter will be greater than

a dc milliammeter (IG). A center-tapped re-

before the ac potential was applied. When the

sistor, RM, is shunted across the milliammeter.

ac potential causes the bias voltage between the

The load, resistance RL, is connected between

control grid and the cathode to become more

the center tap of the transformer (momen-

negative on the other half cycle, the resistance

tarily neglecting resistor RM) and the cathode

of the tube under test will increase, plate cur-

of the rectifier, as in common full-wave recti-

rent will decrease, and the deflecting force on

fier circuits. When plate 2 is positive with re-

the meter pointer will be less. With unbalanced

spect to the cathode, electrons flow through the

current flow through the meter on adjacent half

upper half of resistor RM and through RL, to

cycles and consequent unequal forces applied to

the cathode, causing the meter pointer to de-

the pointer of the meter, the deflection of the

flect in one direction. When plate 1 is positive

pointer will be proportional to the difference

with respect to the cathode, electrons flow

between the currents. Since the difference be-

t h r o u g h the lower half of resistor R M a n d

tween the currents was created by the ac poten-

through RL to the cathode, causing the meter

tial applied between the control grid and the

pointer to deflect in the opposite directior With

cathode, the meter pointer will indicate the

the load resistance fixed and with equal forces

plate current changes produced by the applied

acting upon the meter each time the tube con-

grid voltage change. The meter, therefore, will

ducts, the meter pointer will indicate zero. The

indicate the mutual conductance (para 10a) of

pointer cannot follow to

oprent variations

the tube under test.