Radio broadcast .. (1922-30)

Revamping the Silver Super-Heterodyne Zfi&DEI .0075 .0075 B-MS FIG. 8 Two additional stages of resistance-coupled amplification may be connected in place of the ordinary audio-frequency amplifier specified in the circuit volume of two transformer-coupled stages. This is of no very great importance, however, since the volume obtained from the set is in practically all cases, very much more than will be needed for good loud-speaker operation. It is suggested that lavite resistances of about 48,000 ohms be used as the platecoupling resistances with grid leaks of from j to I megohms. It will be advisable in this case to leave out the jacks in this amplifier and use either the detector output or the full two-stage amplifier output, as is shown in the figure. In this case, a C batten will be necessary only on the last audio stage, where it should be of approximately 45 volts. This is because the effective plate voltage on the first stage is only about 30 to 40 volts, whereas the effective plate voltage on the last stage is very nearly up to the full 90 of the B battery. This will be made clearer when it is realized that a 48,000 ohm resistance is in the plate circuit of the first audio stage which cuts the B voltage to approximately J that of the full plate potential. Only a loud-speaker or a pair of phones is in the plate circuit of the last audio stage with the result that practically all the B battery voltage is applied directly to the tube. SHORT WAVE RECEPTION 'IPHERE is a growing interest in a really * sensitive receiver for operation on the new low broadcasting wavelengths in the neighborhood of 100 meters. The wavelength range of the oscillator used is about 150 to 550 meters, which is more than ample for the entire broadcasting wavelength band. This oscillator range will per mit reception over a range of from slightly below 150 meters to about 600, by using the lower heterodyne point at the upper end of the range, and the upper points at the lower end of the range. It is also possible to use a harmonic of the oscillator to perform the heterodyne function. If the first harmonic, or half the wavelength of the oscillator is used, it means that the range of the oscillator, using this harmonic, would be from below 75 meters to nearly 300 meters. If it is desired to receive a 100 meter signal, the oscillator dial may be set at either of its points where a 200 meter station may have been heard. Then the harmonic will bear the proper relation to the 100 meter signal to create the necessary beat with it. This, of course, is general, but it indicates how the set would be operated. The loop circuit would have to be changed for this work, the loop being cut to about four turns. It may be rather difficult to employ the split loop feature at these waves also. If an antenna is used, the coil to replace the loop may consist of about 20 turns of No. 16 or No. 18 DCC wire, on a three or four-inch form. The antenna coil should contain three to eight turns, depending upon individual conditions. If a set is to be built for short wave work only, the oscillator coils could be wound with fifteen turns each in L.2 and 1,3, and about six or seven turns of heavy wire in Li. SHORT WAVE R. F. AMPLIFIER UNDER certain conditions the experienced fan may find it desirable to add additional R. F. amplification to either of the receivers. A condition which would