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Since the filament transformer has no center tap, a reference must be created, that is, a virtual center tap. Two 10 ohm 20W resistors are placed in series across the filament transformer, and the common filament return point, which is the modulator B- point, is derived from the junction of the two resistors. This allows both the grid and the plate current to flow equally from both ends of the filament. If this were not done, one side of the filament would then have to serve as the common point, and the unbalance would cause a 60Hz component to be impressed in series with the grid and plate currents, causing noise in the modulator's output. The large mica capacitor is connected from the virtual center tap to ground.
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