Radio Broadcast (May 1928-Apr 1929)

NOVEMBER, 1928 A MODULATOR FOR THE 1929 SHORTWAVE TRANSMITTER 39 a 6-voIt storage battery, which is necessary for the microphone in any case. The r.f. amplifier tube in the October article and the modulator tube in the present set-up may be a 1 12-, 17 1-, or 210-type tube, or a corresponding a.c. tube. The UX250 may be used but is not recommended. It is possible to use a.c. tubes in place of the 210 tubes recommended, but make sure that they are of the "heater" type, such as the Arcturus tubes or the UY227, and not of the "thick-filament" type represented by the UX226. Care must be taken to keep the filament leads clear of the grid circuits and even with these precautions the hum problem is apt to be bothersome. With batteries on the tubes mentioned above this difficulty is removed. Still another combination is possible, namely to operate 201 atype tubes in the first sockets and 210-type tubes in the Amp. and Mod. sockets with a.c. on all the filaments. This is done by connecting the 201A filaments in parallel and running them through two equal fixed resistors to posts D and F of the October set-up. If the October set alone is used for c.w. these resistors must produce a drop of 1.25 volts each at a current of \ amp., therefore, they must have a resistance of 2.5 to 3 ohms. If the complete set-up, i.e., oscillator, Cfpse\ 7* \L/Ant D.C.orA.C. A.C.or D.C. FIG. 3. THE SCHEMATIC DIAGRAM OF OSCILLATOR-AMPLIFIER FIG. 2 Diagram A {left) : schematic showing constant-current modulation principle. Diagram B {above) : circuit used in set. A by-pass condenser between B and the plate side of R.F.C. would improve results. amplifier, and modulator is being used the current will be 5 amp. and the resistors may have a resistance 1.25 to 1.5 ohms each. A pair of \ or 5-ohm filament-ballasts resistors will do very nicely. It is advisable to twist all a.c. leads into pairs and to inspect the various center-tapped resistors to make sure they are in good condition. An open resistor will cause considerable noise; an off-rating one will produce less noise that is still quite noticeable. THE UX-2 IO TUBE I F ONE uses 1 12 or 171-type tubes in the r.f. ' amplifier and modulator sockets it is possible to cut down the plate voltage of the oscillator, thus reducing the drain from the batteries. This should not be carried to the point where unsteadiness results. In my particular set 90 volts at the oscillator handle's a 171-type amplifier running at 300 volts with very fine steadiness. When using the ux-210 tube or equivalent in the amplifier and modulator sockets the oscillator tube should have a plate potential of approximately 180 volts. It must, of course, never show plate-heating to a visible degree. If desired, the feed condenser Cf may be changed in size, provided it is not made large enough to cause difficulty in reaching down to the 10-meter band. The neutralizing con denser Cn must of course be readjusted when such a change is made. Since the 210-type tube is used with the thought of obtaining an increased output it is operated at high voltage and with a plate current of about 40 milliamperes per tube — making 80 for the modulator and amplifier. This current would destroy the windings of the National type 90 choke in the plate circuit of these two tubes, and, therefore, it must be replaced by a more substantial choke which will operate over all the wavebands we are interested in. I can find none of the transmitting-type chokes which will do this, and, therefore, suggest a combination consisting of a single-layer choke in series with a General Radio type 379 T, Aero Products type 248 (transmitter type) or a National Type 90 re-wound with No. 32 or 34 d.c.c. wire. The singlelayer choke, which is substituted in the clip for the former choke, consists of a f" rod of insulating material wound for \\" with a single layer of No. 34 or 36 d.s.c. or s.c.c. wire. The other choke may be mounted on the back of the panel near the amplifier tube and antenna ammeter. Its business begins at 20 meters where the little choke stops. 1 n Fig. 8a other changes are suggested that may be necessary if the 210-type tubes are operated at voltages above 250 — as they usually are since their rating is 350. The condenser C5 should be replaced by a 1000volt Sangamo unit of the largest capacity available. If this cannot be done conveniently a change to the shunt method of feeding (shown in Fig. 8b) is advised. This may be necessary in any case if voltages above 400 are used. Here Cs replaces C5. The 250-micro-microfarad plug-in condenser FIG. 4. FRONT VIEW OF MODULATOR UNIT {Cs) used to load the amplifier plate circuit will not survive with the 210, therefore one of two things may be done. Either the coil inductance must be increased or else the 250-mmfd. tuning condenser C3 should be replaced by a 500-mmfd Equitune. The dial degrees of the two tuned circuits, i.e., the oscillator and the amplifier, will then not run together as nicely as was the case before. Of course, if one is really fussy about this point it is possible to use two Equitunes, of the 250 size, set end to end and connected in parallel, one being fixed at maximum and the other variable as usual. Personally, I prefer increasing the inductance. Finally — if someone wishes to use a 210-type tube as oscillator, that too can be done by connecting posts ABC in Fig. 3 to DEF in Fig. 5. The plate potential of the oscillator should not be increased above 180 volts. This combination isn't bad at the upper wavebands but offers some slight difficulty at 10 meters. At present I cannot give the exact dimensions of the somewhat smaller coil that will be required. The diameter will be .ibout x" smaller than shown in the October article. The other wavelengths will shift somewhat but not badly, because of the large condenser which is used. KEYING AND OPERATING WITH VOICE \TO MATTER which sort of operating is to be taken up the first job is to secure a steady output. Some suggestions were given in the October article. To these can be added the fact that it is of comparatively little importance what plate voltage is used as long as the plates remain at a sane temperature. For the oscillator this means no visible color, for the amplifier it means a red that is not too violent. The type of tube used and the recommendations of the maker should be considered. It is helpful to listen to the un-modulated output with a little "breadboard" receiver using the circuit of Fig. 6. With a 199 tube and a 22-volt battery the whole thing can be put on a 7" x 10" base, including a 4|-volt C battery for the filament supply. Shielding is neither necessary nor desirable for such a device; it is a nuisance in fact. When the note seems O.K., and free from 60-cycle ripple, one may key slowly and then proceed with voice-modulation. For this the pick-up receiver is stopped from oscillating. "PERFECT MODULATION" THE voice-input system shown will do good work if given a chance. There are several ways of making the adjustments. One is to have an assistant speak into the mike — preferably reading steadily from a book — while various things are changed and the results noted as the signal is heard