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26 AMATEUR TALKING PICTURES
we can possibly dispense with the use of an output transformer. The resistance of the speech coil of the loud speaker should first be ascertained. Usually this will be of a similar order to that of the speech coil of the recorder, say, 2,000 ohms. For the purpose of comparison it may be stated that the speech coil of the ordinary moving coil loud speaker is usually of the order of 10 ohms.
Even with sets supplying moving iron loud speakers an output transformer will usually be found advantageous. A good plan in adopting a transformer is to make an arrangement with a radio dealer to obtain the loan of one for trial and then try a few experiments with those having slightly different ratios.
If the radio set does not possess "pick-up terminals" we must proceed a little more carefully.
The input leads from the secondary of the microphone transformer will usually be applied to what is known as the "detector valve" of the radio set. A detector valve used in radio works under different conditions to those required for amplification. The detector valve is operating at little or no grid bias. For amplification purposes we require a negative grid bias depending upon the type of valve which can be determined by reference to the data slips supplied by the manufacturers. Frequently it is of the order of one and a half volts.
The best means of making connections to a radio set is to obtain what is known as a pick-up adaptor. This consists of a device which is plugged into the valve holder of the detector valve. It has three pins, the fourth is omitted, and in the normal way couples aerial tuning circuit to the grid of the valve.
A negative bias may be applied to the grid of the detector valve, as in Fig. 12, where are seen the microphone and its battery and transformer. The secondary of the input transformer has one lead connected to the grid of the detector valve, and the other passes through a grid bias battery having its positive terminal connected to the negative "LT" supply or earth.
All this can be accomplished simply by the use of the adaptor such as is shown to the right of Fig. 12. The various