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304
Radio Broadcast
FIG. 2
The Interior of Mr. Zieger's set. Note the rear the neat wiring, and the placing of the
to the number of reflexed stages. That this is so was demonstrated quite conclusively in Mr. Zeiger's set, which was much more unstable than a three-tube set recently brought to the attention of this department. The various adjustments of tubes, rheostats, etc., mentioned above, were necessary in order to stop self-oscillations, which adjustments, however, caused the tubes to be operated in such a manner that they neither detected nor amplified to the best of their ability.
For this reason, we advise our readers not to attempt the building of a four-tube set duplexing all amplifying tubes, unless he possesses infinite patience and experimenting facilities— and luck! Our advice is: Build a four-tube set (or rebuild your present one if you are having difficulty with it), but rather than reflex through three tubes, reflex through only two of them, employing one stage of straight audio amplification. Such a set would use the standard three-bulb Grimes circuit as shown in the April, 1923, number of RADIO BROADCAST*, with a single stage of audio, added according to the directions given in the amplification article appearing in the July issue. The four bulbs would then be, two reflex, detector and one
straight audio. This set will be much more stable than the average four-tube, all reflex set. It will probably give even louder signals than the full reflex set working at its best, for one step of straight audio amplification is always superior to a stage of reflexed audio, and the third step of R. F. is rarely of much advantage.
A PORTABLE SET FOR AN
connections, transformers
F
OPEN ANTENNA
IGS. 3 and 4 show interesting views of the three-bulb set made by Mr. Leonard H. Searing, in which, for the greater part, he followed the circuit shown in the April, 1923, RADIO BROADCAST. His main variation from the original diagram is the use of a variocoupler which permits the set to be operated from an open antenna rather than on a loop. The primary of the variocoupler is in the aerial circuit with a variable condenser in series with the ground lead. The secondary is connected in place of the loop — a simple and self-evident change.
However, a loop may be used with Mr. Searing's set, and in order to facilitate the change from open antenna to coil antenna (loop), a standard double-circuit jack has been placed across the secondary of the variocoupler (be
* Back numbers of the magazine may be had from the publishers at twenty-five cents each. A portable three tube Grimes set, for outside antenna or loop, built by Mr. Searing