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326
Radio Broadcast
FIG. I
The "Fundamental Circuit." Build your set with this as a basis if you do not care to follow explicitly the instructions given by the writer, or if you cannot obtain the parts designated. You may use such rheostats, jacks, transformers, etc., as you have on hand and as are within the latitude always allowed the constructor
this particular, and rather unusual connection, there exists also, perhaps, a neutrodyning effect — the system applied so successfully to the elimination of capacity phenomena in radio frequency amplification.
THE CIRCUIT
WE HAVE drawn up for the benefit of our readers, two circuits, the fundamental circuit, Fig. 1, and the specific circuit, Fig. 2. The fundamental circuit, which we shall first consider, is the basis on which the majority of enthusiasts, who are unable to obtain the exact instruments used by Radio Broadcast in building the set, must work.
Ti and T2 have been described in detail in the November Radio Broadcast, and a very interesting variation, the use of spider-webs, is covered in the " R. B. Lab." Department for this issue. If a homemade T2 is used, the tentative condenser, C3 capacity, .0005, will be necessary. If a Ballantine Varioformer is employed, this condenser is done away with. The audio transformers, T3, T4, and T5 may be any reliable make, such as Acme, Federal, Amertran, etc., with a ratio of approximately five to one. The same make or type of transformer need not be used throughout the circuit. " Det." represents any good crystal detector.
C2 is the anti-capacity condenser, and should be as small a capacity as is effective. Generally a .0005 Micadon suffices. This condenser, incidentally clears up other objectionable noises,
and noticeably reduces A. C. induction from near-by lighting wires (electric lights in the vicinity of the operating table, etc.). In a few instances, and with some tubes, this condenser may be unnecessary.
No rheostats are shown in the fundamental circuit. If UV-190/s are used, and a steady three-volt source is available, the filament adjustment may be eliminated. If the builder desires to use individual rheostats, one for each tube, they will be connected in the filament circuit at "Y". If a single rheostat is decided upon, as is most likely, it should be inserted at "X". In all cases of resistances, it will be noted that the secondaries of the external audio transformers are brought down to the battery side of the rheostats. This places a desirable negative potential on the grids.
No jacks have been indicated. The constructor may use any type he possesses, or can obtain conveniently, filament control or otherwise. Various types of jacks have been pictured and described in the April issue of Radio Broadcast. While jacks are of course advisable, the set will function at all times on the last amplifying stage.
THE SPECIFIC CIRCUIT
THE set as built by Radio Broadcast employed the Ti exactly as described in the November issue. T2 is a Ballantine Varioformer. T3, T4, and T5 are Amertran transformers. Standard sockets (De Forest) were