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In the R. B. Lab.
305
fore it connects to the secondary condenser). The inside terminals of the jack run to the variocoupler, while the outside prongs are connected across the secondary condenser from which they run to the potentiometer and grid of the first tube. The loop may thus be plugged in, a process which automatically disconnects the variocoupler. It might be more convenient to mount this jack in the top of the cabinet, making it possible to use a loop with a plug in the bottom of its upright support (after the manner of the De Forest reflex), thus doing
The interior of the three-tube set.
FIG. 4 Plenty of room here, and no crowding of parts
away with the separate stand for the loop.
Mr. Searing obtains excellent results with this set, his only complaint being that it tunes rather broadly. This is surprising, for the use of the variocoupler should result in a very selective set. It is possible that Mr. Searing is using an antenna of unusually high resistance (of small-size wire). This will broaden tuning, and in several cases which have been brought to the attention of this department, it has decreased the selectivity of good receivers. No wire smaller than No. 14 should be used, and No. 1 2 is to be preferred.
1 1 will be noticed, from Fig. 4, that the builder has not crowded his apparatus. Plenty of space in the Grimes circuit lessens the chance of undesirable complications.
NEUTRODYNINGTHE DUPLEX
AjAIN we emphasize the prevailing fault of reflex circuits— the tendency towards self oscillation. In an endeavor to overcome this, Messrs. John C. Stick and N. P. Moerdyke have evolved about the best Duplex circuit we have yet seen (Fig. 5), in which they have combined three effective methods for the reduction and elimination of self
oscillation. First of all, they have minimized the feed-back possibilities by not over-doing the reflexing, having thrown back the energy through only two tubes, leaving the first tube to function as a straight radio-frequency amplifier. Secondly, they detect with a crystal, eliminatingthe complications which attend bulb detection in such circuits, and which caused the De Forest Company to adopt similar means of rectification. Thirdly, they applied the neutrodyne principle, and, "obtained a great improvement over the straight Grimes hook-up. As in the neutrodyne, the tendency to oscillate, and the presence of body capacity, were completely
, NEUTRALIZING CONDENSERS
FIG. 5
The best Inverse Duplex circuit we have yet seen. The neutrodyne condenser helps smother self-oscillations