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Radio-Frequency Amplification From the Ground Up
503
FIG. 6
This special transformer is used to couple the last R. F. tube to Detector Tube No. 2 in Fig. 5, and provides a sharp resonant point for the radio-frequency circuit. The walls of this transformer may be made of bakelite or hard rubber. The primary is wound with 200 turns of No. 29 D. S. C. and is separated from the secondary, which has 1500 turns of No. 36 D. S. C., by several layers of empire cloth
cuit are the Radio Corporation's type UV-i7i6, (Fig. 7) which may be used to cover a very broad band of frequencies. Each stage of R. F. is shielded by rather heavy metal shielding.
Another advantage of the arrangement shown in Fig. 5, lies in the fact that the tuned R. F. coupling transformer (S. T.) which determines the frequency at which the intermediate frequency circuit must function is placed directly before the second detector tube rather than directly following the first detector tube, is as the case in Fig. 4. The construction of this transformer is indicated in Fig. 6. Its advantage lies in the fact that whatever losses are brought about in this circuit may be sacrificed with less ultimate loss after the amplification has taken place.
A SUGGESTION
FOR those who would use the superheterodyne and would care to cut down the expense of its operation, we would suggest a method for reducing still further the number of tubes required. We have not had time to give this method the trial which usually precedes the publication of information in RADIO BROADCAST and therefore refrain from publishing the circuit we have in mind, but there is no apparent reason for any difficulty being experienced with it.
Where such high amplification is used, there seems to be no reason for overlooking a good crystal detector to follow the R. F. amplifiers. That would eliminate one tube.
There seems to be much logic in the Grimes method of reflexing and if Armstrong's contention is true, the current flowing in the first two R. F. tubes is very small indeed. Why not go back to them, then, for the audio-frequency, thus doing away with two more tubes without a great loss in over all performance.
In suggesting this arrangement, we have not lost sight of the fact that a tube used in a reflex arrangement may not be serving at its best' as a radio or audio-frequency amplifier, but this loss does not seem to be a very serious matter. For the experimenter who is anxious to produce great volume, the use of three stages of audio-frequency is to be considered. In this last arrangement, however, a great variety of difficulties are likely to arise.
The production of uniform tubes for use in amplifying circuits operating on very low filament consumption and amplifying transformers that will cover a great band of frequencies make it possible for us to look for some very marked improvements in reception. To the best of our knowledge these suggestions have not been made before, and RADIO BROADCAST would like to hear from those who attempt to put them into practice.
FIG. 7
The UV-i7i6 R. F. transformer has a wavelength range of from 5,000 to 25,000 meters. It may be used at the frequencies found in the intermediate frequency circuit of the super-heterodyne and produces a much greater amplification, per stage, than is possible with the resistance-coupling