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The "lab" department has been inaugurated by Radio Broadcast in order that its readers may benefit from the many experiments which are necessarily carried on by the makers of this magazine in their endeavor to publish only "fact articles" backed by their personal observations.
THE LATEST REPORTS ON THE GRIMES CIRCUIT
IT IS doubtful if any other circuit made public in recent months has received so enthusiastic a welcome by the broadcast fan as the Grimes Inverse Duplex. The difficulties encountered in making this circuit function as it should, have not daunted the experimenter, and every mail received by Radio Broadcast contains new evidence of his unflagging interest in the circuit.
The Grimes circuit is a queer one. It is a puzzle, and deserves all the light that experimentation can throw upon it. Our readers who have experimented with this set, in either the three bulb or four bulb form, are divided sharply into two factions — those who claim it is the best circuit in the world, capable of the most remarkable results, and those who cuss it causticly as a set that will howl, squeal and do everything but receive signals.
However, the circuit will work, and we are going to let our successful readers give a few pointers on how it is done — including, here and there, a few words of our own.
A FOUR-TUBE SET
FIGS, i and 2 show the four-tube set constructed by Mr. Martin A. Zeiger in his endeavor to duplicate the installation of Mr. Shalkhauser, the winner of Second Prize in Radio Broadcast's last " How Far Have You Heard" contest. The corrected circuit for Mr. Shalkhauser's set was published on page 142 in the December number, while other details will be found in the September issue.
These photographs of Mr. Zeiger's set are particularly interesting in that they show very clearly an accepted way of placing the parts, mounting, etc. Fig. 2 is also illustrative of the care and neatness in wiring which so often determines whether or not a receiver will work.
In order to make Mr. Zeiger's apparatus function, it was necessary to add two small fixed condensers (.001 mfd,) across the secondaries of the audio transformers nearest to the detector, as was recommended last month in reference to the corrected diagram. The selection of tubes was found to be eccentric, to say the least, and an indiscriminate juggling of a half dozen UV-201-A's was necessary before results were at all satisfactory. Filament control was such as to necessitate separate rheostats on each tube, no two tubes being burned at the same brilliancy.
The tendency towards self-oscillation in a reflex set (its predominant fault) is in a direct ratio to the number of tubes, or, more correctly,
fig. 1
A compact four-tube Grimes set. Potentiometer, condenser and four rheostats are the only controls