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1338 HANDBOOK OF PROJECTION FOR
A short or ground will show up either on the milliammeter or in the form of blown fuses. If you suspect a short, it is only necessary to test from each socket to all points it connects to — to all other points in the amplifier filament circuit. From any one socket there are only two points which will give you a buzz, namely, from one hole to the next place further on in the circuit, and from the other hole to the next place in the other direction.
For example, testing from the center socket to the plus 12 terminal should give no buzz. If it does, then there is a shunt path of some sort around the socket. Had the tube been left in its socket just this sort of condition would obtain. That is why we remove them before testing — that and the further fact that we could not test the holes with the tubes in.
As to a ground, every projectionist knows how to test for that. See page 356, Volume 1, but it must be remembered that the negative 12 terminal is normally grounded by intention, hence you will get a buzz from one hole in the socket on the right of the amplifier to ground. That is as it should be.
SHORTING OUT RESISTANCE.— In case of emergency you can get along with the 4-ohm resistance shorted out, but it is not a good condition. Better keep one of those resistances in stock. It is possible to connect two flashlight bulbs in parallel, and use them in lieu of the 4-ohm resistance, as such bulbs usually have about 10 ohms resistance. However, we do not recommend such makeshifts. Better have spare parts in stock.
In lieu of the 24-ohm resistance it is possible to use two of the flashlight 10-ohm bulbs connected in series,