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(i) The surfaces of the valve and of the input resistor close to the value of the free grid potential. Under
must be kept free from all sources of leakage. The these conditions care must be taken to ensure that
surface of the BM30 is silicone coated to render it only negative signals are applied to the grid and,
moisture repellent, but that portion between the top even so, relatively unstable operation may result.
cap and the external guard ring should not be handled
unless it has first been wrapped in clean dry paper. ADVANTAGES OF THE BM30
If the silicone coated surfaces of the valve or the
resistor should become contaminated by finger- The BM30 incorporates the following advantages
prints or dust, etc., they may be cleaned with which promote maximum control grid insulation and
absolute alcohol applied by means of a clean high stability operation:
duster. (i) The electrodes are rigidly mounted. In particular
(ii) Operate the valve and input resistor in an enclosure an unusually rigid mounting of the control grid is
containing a drying agent such as phosphorus pent- employed. Microphony has thereby been con-
oxide or calcium chloride. Silica-gel may be used siderably reduced.
when the valve, resistor, and drying agent are (ii) The control grid mountings are designed to pro-
effectively sealed from the atmosphere. duce, and maintain, very high insulation of the
(iii) Operate the valve in a light-tight enclosure, prefer- control grid. The glass end-sleeves prevent the
ably blackened internally, to limit photoelectric deposition of a conducting film between the control
currents from the control grid and the getter film to grid and the remainder of the assembly.
those produced by the light from the heater and (iii) As an indirectly heated valve:
the cathode. (a) Its cathode is an equipotential surface.
(iv) Employ adequate electrical and magnetic screening (b) Its cathode operates at a lower temperature
for the valve and input circuit. A copper or than that of a directly heated valve, thereby
copper-lined chamber is usually adequate. greatly reducing the photoelectric and heater
emission effects.
(v) Ensure that the potential at any point of the heater (c) It has a lower free grid potential (i.e. the poten-
is not more positive than that of the cathode, e.g. tial which, under the given conditions of potential
in direct current operation of the heater connect
the heater positive terminal to the cathode. In of the other electrodes, the control grid acquires
when disconnected from any external source of
alternating-current operation of the heater main- electricity) and one whose value is able to be
tain the centre-tap, of the 4-volt heater supply, 4
volts negative with respect to the cathode. controlled, within limits, by adjustment of the
heater /cathode potential difference.
(vi) Avoid contact potentials between dissimilar metals (d) Its cathode has a larger space charge-which
in the input circuit. reduces the effect at the anode of emission
changes of the cathode; and, owing to its greater
(vii) Use high capacity accumulators to supply the rigidity of mounting, is subject to less micro-
electrode voltages. phonic disturbance.
(viii) Whenever possible use high-quality wire-wound (e) It has a greater range of control grid potential
over which its control grid current lies within
resistors. If variable resistors are to be used en- the specified range-thereby enabling the accur-
sure that they are wire-wound and are free from ate measurement of a wider range of input
contact troubles. signal.

(ix) Do not apply the anode and screen potentials until (f) Suitable heater/cathode potential difference can
after the cathode has reached its operating tempera- be applied to neutralise the effect upon the con-
ture, and always remove these voltages before the trol grid current of emission from the heater.
heater supply is turned off. Failure to do this (iv) It employs internal and external guard rings, which
causes momentary operation of the cathode under serve to reduce the effect upon the grid circuit of
temperature limited conditions which tends to the migration of wall charges from the glass
destroy its emitting surface and, as this surface is envelope to the electrodes.
only slowly replenished on such low temperature
operated cathodes, produces increased long-term (v) The glass envelope is silicone coated to render it
drift. moisture repellent.
(x) For maximum stability the valve should be oper-
ated under conditions which, at the working point, OPERATION OF THE BM30 AT VARIOUS
produce a flat portion of the control grid current/ ELECTRODE POTENTIALS
control grid voltage curve. In determining the optimum operating conditions for
Minimum control grid current may be achieved the BM30 the effects upon the characteristics of operat-
by operating the valve at a control grid voltage ing the valve at various electrode potentials were con-
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