To study the effect of the grid condenser you can arrange switches so as to insert this condenser and its leak and at the same time to cut out the C-battery. Fig. 115 shows how. You can measure the gain in audibility at the same time.
Pl. X.–Audio-frequency Transformer and Banked-wound Coil. (Courtesy of Pacent Electric Co.)
219After learning to use the audion as a detector, both by virtue of its curved characteristic and by the grid-condenser method, I would suggest studying the same tube as an amplifier. First I would learn to use it as an audio-frequency amplifier. Set up the crystal detector circuit. Use your audio-frequency transformer the other way around so as to step up to the grid. Put the telephone in the plate circuit. Choose your C-battery for amplification and not detection and try to receive.
You will get better results if you can afford another iron-core transformer. If you can, buy one which will work between the plate circuit of one vacuum tube and the grid circuit of another similar tube. Then you will have the right equipment when you come to make a two-stage audio-frequency amplifier. If you buy such a transformer use the other transformer between plate and telephones as you did before and insert the new one as shown in Fig. 116. 220This circuit also shows how you can connect the switches so as to see how much the audion is amplifying.
The next step is to use the audion as an amplifier of the radio-signal before its detection. Use the proper C-battery for an amplifier, as determined from the blue print of the tube characteristic. Connect the tube as shown in Fig. 117. You will see that in this circuit we are using a choke coil to keep the radio-frequency current out of the battery part of the plate circuit and a small condenser, another one of 0.002 mf., to keep the battery current from the crystal detector. You can see from the same figure how you can arrange the switches so as to find whether or not you are getting any gain from the amplifier.
221Now you are ready to receive those C-W senders at 275 meters. You will need to wind another coil like the secondary coil you already have. Here is where you buy another condenser. You will need it later. If before you bought the 0.0005 size, this time buy the 0.001 size or vice versa. Wind also a small coil for a tickler. About 20 turns of 26 wire on a core of 3-1/2 in. diameter will do. Connect the tickler in the plate circuit of the audion. Connect to the grid your new coil and condenser and set the audion circuit so that it will induce a current in the secondary circuit which supplies the crystal. Fig. 118 shows the hook-up.
