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The sensitivity of d.c. amplifying circuits with FP-54 tubes is limited by the stability which can be obtained. In single-tube circuits it is necessary to balance out fluctuations caused by changes in battery e.m.f. and in filament emission. Soller's circuit for balancing out e.m.f. changes has been studied in detail and a modified circuit has been developed in which the adjustments are less critical and which allows balancing out of changes in filament emission also. The latter is accomplished by balancing the plate current against the current to the space-charge grid. Details of construction and manipulation of the new circuit are given, as well as data on its stability.
DuBridge et al. (1933) studied this question.