Richardson's handbook of projection (1930)

MANAGERS AND PROJECTIONISTS 1375 A ground in the rectifier filament circuit shorts the plate supply. The rectifier plates will be burning up, no current showing on the meter and the amplifier tubes remaining cool. The effect is the same as with a punctured filter condenser. The filaments are the positive source of the high voltage and are directly connected to the first bank of the filter condensers. A shorted filter choke, or an open in the wiring to the filter condensers, providing the path from the rectifier filaments to the output transformer is unbroken, and that the path from the center of the high voltage transformer to the ground also is continuous, will cause no trouble, as this amplifier can be supplied with almost unfiltered, rectified current, and yet not produce sufficient hum to be objectionable. Also there will be no hum produced in the 41 amplifier by reason of poor filtering in the 42, by reason of the fact that the 41 itself has an elaborate filtering system. Opens in the grid circuits set up hum and high plate current. Opens in the plate circuits obviously stop the current flow to the tube, or to the tubes, as the case may be. No plate current flows when the bias resistance becomes open circuited, since it really is a part of the plate circuit. It forms the path from the filaments back to negative B. Troubles which may occur in the wiring of the 42 amplifier is quite easily repaired, but when some of the parts themselves go bad it becomes quite a difficult problem. Other parts cannot easily be substituted in this amplifier. Not only must any replacement have very nearly the same values of the part replaced, but also they