Radio Broadcasts Knock Out Four Tube Receiver

By JOHN B. BRENNAN

I AST year Walter Van B. Roberts did a considerable amount of independent investigating with various circuits and about the first of this year, evolved the arrangement which has gained unusual popularity as the Roberts circuit. That circuit, as most everyone now seems to know, if we are to judge by the tenor of the enthusiastic correspondence piling into the office daily, is sensitive, selective, uses a minimum of tubes for a long range receiver, and does not radiate. The extraordinary efficiency of this set is caused bv its use of reflexing, regeneration, and proper tube-neutralization. Mr. Roberts described his set in this magazine for April and May. Two construction articles appeared on the receiver, one by Zeh Bouck telling how the circuit might be used for very short-wave reception, and the other by J. E. Roberts describing a layout with three tubes in our August number. The receiver-described here is without doubt the best, for the constructor who is looking for results, that we have ever seen.—The Editor.

By developing his two-tube reflex circuit, Mr. Walter Van B. Roberts has contributed to radio a I receiver of inestimable value and importance. The claims for this receiver made by Mr. Roberts have been fully borne out by the many reports from those who have lost no time in building it, as well as by the further research work conducted by Radio Broadcast's laboratory.

Combining the advantageous features of radio- and audio - frequency amplification through reflexing, regeneration, and neutralization, this circuit is particularly desirable since it does not radiate.

However, an attempt is not made here to repeat the theory of the operation of this circuit as given by Mr. Roberts (Radio Broadcast, April and May, 1924), but to supply the radio fan with the data sufficient to enable him to construct this receiver with the addition of an efficient push-pull amplifier.

The ordinary amplifier unit will not consistently operate efficiently with this receiver unless special corrective features are incorporated to control the resultant distortion, howling, or overloading.

The push-pull amplifier unit has been found to supply the desired stability, faithfully amplifying over the average audio-frequency range without distortion.

When a signal is applied to the grids of the lubes (Fig. 1) by the inductive relation of the primary to the secondary, one grid becomes positive while the other is negative. Naturally an alternate push and pull action of the currents flowing between the plates of the tubes through the primary winding of the output transformer takes place. When the current is increasing in one section of the windings and decreasing in the other it would be expected that the resultant current in . the secondary would be small. But the natural polarities of the two sections oppose each other, for the B battery current flows through the end leads of the coils out through the center tap. This causes the two currents induced in the secondary to add up, and the maximum current is delivered to the loud speaker as alternating current.

In the ordinary type of amplifier the current in the plate circuit is pulsating "direct current" with the loud speaker winding in series. When a heavy current is passed through this circuit the diaphragm does not readily respond to the minute changes of current intensity.

By inductively coupling a secondary to the primary and removing the loud speaker connections to the secondary terminals (as in the push-pull unit), the pulsating direct current is transformed to a modulated alternating current.

In the push-pull amplifier, we have a primary-delivering energy to two tubes through a split secondary, a C battery to supply the proper negative bias to the grids, a split primary delivering the sum of the output of the two tubes

Push Pull Amplifier Circuit

A standard push-pull amplifier circuit. Usually this unit is used as a second stage but with the Roberts circuit the first audio stage is reflexed and its output passed into the circuit shown here. Exceptional volume and tone quality result

T- CONNECTS TO EITHER K- Oil A+ AS CASE MW EE rn PRE- x,u . x CEDING AMPLIFIER OR RECEIVER - -

A standard push-pull amplifier circuit. Usually this unit is used as a second stage but with the Roberts circuit the first audio stage is reflexed and its output passed into the circuit shown here. Exceptional volume and tone quality result in pulsating direct current, and the secondary delivering alternating current to the loudspeaker. Each one of these features helps in its own way to prevent the possibilities of distortion and overloading.

The value of the C battery varies with the value of the B battery used as the following table shows:

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