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The curves of Fig. (10) show the
corresponding changes produced in the
free grid potential of valves A and B
as the heater/cathode potential differ-
ence is varied. The continuous lines
denote the results obtained at V a = 6
volts, V g1 = 4 volts, V h = 4 volts and
the broken lines show the values at
V a = 6 volts, V g1 = 6 volts and V h =
4 volts. In all cases the free grid
potential is seen to be of constant value
for values of V k, h- between 3 and 10
volts and that as V k,h- decreases
below 2 volts the free grid potentials
become variable and more positive.
Stable operation is seen to occur at the
recommended operating condition of
V k,h- = 4.
Fig. (11). The Balanced Bridge Circuit.

A review of the above results indicates (Fig. (6)) that The circuit used is shown in Fig. (11) in which all
variation of anode voltage alone, within the range con- the required voltages are seen to be supplied by one
sidered, causes little change in the control grid current. 12-volt battery (B) via a resistance network. The pro-
Such does not apply, however, in the case of variations cedure for stabilising the circuit is to adjust the four
of the screen and heater voltages either of which, as is resistances R 2, R 3, R a and R n ohms, in terms of the
shown in Figs. (7) and (8), can drive the control grid operating conditions and corresponding valve character-
into the state of free grid potential and then produce a istics, such that the galvanometer current shall be zero
reversal in the direction of the control grid current. and constant for small variations in heater current
Figs. (9) and (IO) show how the value of V k, h- can be caused by varying either R 4 or the battery voltage.
used as a means of controlling and stabilising the con-
trol grid current. The vital effect of the free grid
potential of the valve, upon the characteristics of the Balancing the Circuit
valve, is also noted and it is seen that the excessive The procedure for setting up and balancing the
reverse control grid current produced when the control circuit is as follows:
grid is operated at a potential more positive than the
appropriate free grid potential renders the valve un- 1. Measure the values of the heater current (I h). anode
serviceable as an electrometer valve, i.e. for electrometer current (I a) and screen current (I g1) for the valve
operation the control grid potential of the valve, at the when operated at its rated voltages of V a = 6
working point, must be more negative than the free grid volts, V g1 = 4 volts, V g2 = -2 volts, V h = 4
potential of the valve under such conditions of anode, volts and V k, h- = 4 volts.
screen, heater and V k,h- potentials. [Let the measured values be I h = .125 amp., I a = 116 µA
and I gl = 200 µA].

THE PERFORMANCE OF THE BM30 IN A 2. Set up the heater circuit with a suitable centre-
BALANCED BRIDGE CIRCUIT tapped resistor R 1, of approximately 10,000 ohms,
shunted across the heater terminals and to be in
The Circuit series with R 3, R 4 and the 12-volt storage battery.
The sensitivity of a direct-current amplifier using an Select R 3 and R 4 such that R 4 may be adjusted to
produce a steady heater voltage of 4 volts and may
electrometer valve is often limited by the stability of the also be used to produce the required variations of
valve and of the batteries which supply its power. Valve the heater current.
stability had been carefully considered in the design and
construction of the BM30 and circuit conditions to [If I h = 0.125 amp. and V 2 = 8 volts the value of R 3
8
effectively counteract the drift and fluctuations of the for balance will be = - = - = 64 ohms. The
V 2
battery supply were now considered-with the result 0.125
that it was decided to modify the Barth-Penick exten- value of R 4 will then be such as will drop the voltage by
I h
sion of the DuBridge and Brown circuit so as to render which the battery p.d. exceeds 12 volts and capable of
it applicable to the indirectly heated BM30. producing the requisite values of heater current variation].
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