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1374 HANDBOOK OF PROJECTION FOR
Should the plate and filament of an amplifier tube become shorted it short-circuits the plate supply. The amplifier tubes remain cool but the rectifiers are burning up. The meter will go off scale.
When the grid and filament became shorted the tubes will draw very heavy current, though nothing like that caused by a shorted plate and filament. The meter will probably go off scale again.
A short in the wiring of the grid and plate circuits will show up the same as a short from the grid to the plate inside the tube, but removing a tube does not restore things to normal. Removing both tubes brings the meter to about 5 mils. This is the current that is flowing through the short, through half of the input transformer, then through the 48,000 ohms resistance and thence to ground.
A ground to the plate circuit between the meter and the tubes will cause the meter to go off scale, and it will remain that way, even with both amplifier tubes removed. The plates of the two rectifier tubes will be burning up.
A ground on the supply side of the meter does not give a reading on the meter. Such a fault may be suspected when the rectifier plates become extremely overheated and the meter reads zero, although the ground is due to a shorted condenser, a small current value will often show on the meter. It may be anything from 0 to 20 mils. A shorted condenser is, by the way, almost always the cause of such a ground.
A grounded filament circuit in the amplifier shorts the bias resistance, in consequence of which the plate current goes up again.