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1370 HANDBOOK OF PROJECTION FOR
gain control switch, or the condenser between the plate of V2 and the grid of V3. (f) An open grid circuit in any of the stages, which same will be accompanied by a hum or growl.
VI draws a high plate current when condenser CI is shorted. This is for the reason that it removes the grid bias by connecting the grid return directly to the filament, instead of the lower end of the biasing resistance R13 and R14.
In the event that resistance R13 becomes shorted, V2 will draw too much plate current. If R14 becomes shorted it also throws off the bias on tubes 1 and 2, but the filament current cannot be kept down to 270 when this occurs. This trouble will, therefore, be immediately indicated. It may be repaired without difficulty.
Grounded plate circuits or shorted bypass condensers in the plate circuit — a shorted condenser here actually grounds the circuit — are the only causes that will give a high meter reading and still no current be flowing through the tube. In all other cases of high current indicated by the meter it will be current flowing through the tube.
Under similar conditions — that is to say the tubes and the filament circuits being in good order — the reasons for low plate voltage are (a) Supply voltage less than 390. (b) Open plate circuit or circuits, (c) High resistance in one or more plate circuits, (d) A resistance accidentally shunted across the meter. In the case of V2, in case the grid becomes grounded, current will be below normal for the reason that there will be excessive grid bias.
When seeking trouble in the plate circuit of V3, bear in mind the fact that a portion of the circuit is included