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238
Radio Broadcast
The one-tube reflex described below.
FIG. 2
It will operate a loud speaker on local stations
For a description of the parts, other than the Variotransformer, and for constructional data, the reader is referred to the November issue of Radio Broadcast. This set can be mounted on a panel, the Variotransformer being made in panelmounting style.
THE ONE-TUBE REFLEX WITH A BALLANTINE VARIOTRANSFORMER
(Report by Dr. L. M. Hull of experiments conducted by him at Radio Frequency Laboratories, Inc., Boonton, N. J.)
THE circuit described in the article "A Single-Tube Reflex Receiver" has been assembled and tested, using as the radio-frequency transformer, T2, a Ballantine Variotransformer, Model 5. Transformer T (refer to Fig. 1 in the above designated article) was constructed for our test circuit according to the specifications supplied by the writer. The transformer, T3, was a General Radio "Amplifying Transformer" having a turn ratio of approximately 4:1. A type UV-201-A tube was used, with approximately 80 volts on the plate.
As a crystal detector we used alternatively an iron point on iron pyrites or a combination of pyrite and ferro-silicon which we have found to possess particularly fine rectifying qualities.
Assuming that the main point of interest is the performance of the circuit containing a Variotransformer relative to its performance with the transformer specified in the article, a second tuned transformer was constructed according to the specifications supplied for T 2, and for purposes of comparison this transformer (which will hereafter be designated the "T2 coupler") was substituted for the Ballantine Variotransformer at given adjustments of the crystal detector. For the reception of signals an antenna and counterpoise were connected between terminals A and G* having in combination an effective capacity of 700-500 micro-microfarads between 300 and 600 meters and an effective resistance varying from 13 to 16 ohms in this wavelength range.
When this circuit was excited by a locally generated modulated radio-frequency E.M.F., the Ballantine Variotransformer produced an appreciably greater amplification than the T2 coupler at wavelengths between 300 and 400 meters, indicated by a louder signal in a head set connected between the jack terminals (see Fig. 1 , page 218) as well as by the production of an audible sound with the Variotransformer from a signal so weak as to be inaudible with the T2 coupler: there was no appreciable difference in selectivity in this range. Between 400 and 500 meters the signal intensity appeared to be very closely the same with either radio-frequency transformer. It was possible, by selecting a highresistance contact on the crystal to force regeneration to the point of strong sustained oscillation with the T2 coupler, thereby increasing the sharpness of tuning above that obtainable with the Variotransformer. The circuit containing the Variotransformer was by far the more stable of the two, since it was found to be impossible to throw the circuit into oscillation with any adjustment of the crystal and
*Shown in diagram on page 218.