In some cases it has been found that an excessive 60-cycle hum was being introduced on the Model 60 from the power line. A single .015 by-pass condenser is connected in this chassis from one side of the line to ground. The hum trouble was corrected by connecting a [bakelite block style] twin No. 3793 Condenser (.015 Mfd.) across the line with the center point grounded to the chassis. It was only on certain power lines where this condition existed. In the great majority of cases, the single .015 Mfd. Condenser is entirely satisfactory.
From the Philco Serviceman – May, 1934.