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3. Join the cathode to the positive terminal of the
heater. The control grid voltage, obtained from the
centre-tap of the heater shunt resistor R 1, will now
be at the specified value of 2 volts negative with
respect to the cathode.
4. Into the screen circuit introduce the resistor R d of
such value as to produce, when conducting the
screen current I g1 amp., a voltage drop V 1 equal to
the difference between the specified operating volt-
ages of the anode and of the screen.
6- 4
V a-V g1
[If I g1 = 200µA then R d = =
I g1 200 x 10 -6
= 1O,OOO ohms].
5. Introduce the resistor R n into the screen circuit and
let it be of such value as to produce a volt drop of
(V 2 -V 1) when conducting the current I g1 amp.
8 - 6
[R n = V 2 - V 1 = = 10,000 ohms].
I g1 200x10 -6
6. Set the anode supply voltage V a(b) to an arbitrary
value between V 2 and V 1 and such that the anode
potential V a will be 6 volts.
[Let V ( ) = 7.02 volts].
a b
7. Select resistor R a to have value such that (V a(b)
- R a.I a) = (V 2 ---- .I gl).
R n
7.02 - 8 + 2
V a(b) - V 2 + Rn.I gl =
[i.e. R a =
I a 116 X 10 -6
= 8,793 ohms].
8. Close the galvanometer circuit and, if current flows,
adjust R a to reduce the galvanometer current to
zero.
9. Adjust R 4 to vary the heater current over a small Fig. (12)
range near the operating point and determine, from
the galvanometer deflection, whether V a(b) is too
high or too low for balance. Results obtained at Balancing Stages (9) to (11).
10. After returning the heater current to its normal 4 The galvanometer was connected so as to give a
volt value, by means of R 4, adjust R 2 to change positive deflection when the anode voltage V a exceeded
V a(b) by a small amount in the required direction V 1, i.e. when V a(b) was too large.
and re-adjust R a to reduce the galvanometer deflec-
tion to zero. On completing the procedure of balancing stages (9)
11. Again vary the heater current and repeat operations and (10) a region of balance was found to occur at
(9) and (10) until a balance is indicated, by the R a = 9,400 ohms.
absence of a galvanometer deflection, when the
heater current is varied by a small amount near the On plotting the stability characteristics at values for
operating point. V a(b) of 6.99, 7.03, 7.07, 7.15, 7.19, 7.3 and 7.4 volts at
corresponding R a values (at zero galvanometer deflec-
Alternatively: tion for I h = 0.125 amp.) of 8617, 9011, 9373, 10055,
10345, 12284 and 13252 ohms respectively, it was found
Construct stability characteristics for the circuit (as shown in Fig. 12) that two of them approached the
showing the galvanometer current (or output) I o as a requirements of the correct curve (i.e. a curve which
function of the heater current I h, as shown in Fig. (12), has zero slope at the operating heater current.
for a series of corresponding values of V a(b) and These were the curves of V a(b) = 7.15 volts and
R a, and ensure that V 2 is maintained constant when- V a(b) = 7.07 volts. Taking the mean of these two
ever adjustments of V a(b) and R a are made at the rated values as the correct value we find that the balanced
value of I h. circuit will have V a(b) = 7.11 volts, whence:

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