Radio Broadcast (May 1928-Apr 1929)

THE EQUAMATIC REVAMPED FOR REGENERATION The Equamatic system is well known to all radio experimenters and home constructors. The addition of one small condenser, C,t, makes a lot of difference in the sensitivity of this circuit — as well as that of any neutrodyne or t. r. f. set Adding Regeneration to Any Herbert Grove ,NE of the commonest ways of finding out whether or not a radio scheme works is the "cut and try" method. This is usually the longest way around an often simple problem, but radio has not yet advanced to the point where one can do all his labor at his desk or drawing board. Some work must be done with a screw driver, soldering iron, and pliers. A little judicious work with pencil and paper, however, will often save hours of labor and promise either ultimate disappointment or success. For example, if you knew that adding a little regeneration to the detector of a neutrodyne, or t.r.f. set, or an Equamatic receiver, would give it sufficient pep to enable it to get that elusive dx, how would you perform this minor operation? Before presenting a system for which we are indebted to Mr. Louis G. King, known to Radio Broadcast's readers as the originator of the Equamatic system, let us look at a few simple diagrams. In Fig. i we have the conventional detector circuit of a receiver — let us say a neutrodyne — in which an r.f. stage precedes the detector. This is the fundamental detector circuit used in such receivers as the Atwater Kent, the Crosley, t.r.f. sets, etc. Following Professor Hazeltine's patent papers, No. 1,648,808, the primary P, will have about 8 turns, the secondary, S. about 60, and there will probably be a bypass condenser,' Q, from the plus 90 lead to one side of the filament circuit, Making Regeneration Simple JK/fANY radio writers have a habit of making very simple things seem most complex. In this article, the originator of the Equamatic circuit, which was first described in this magazine, does the opposite. He tells how in a very simple fashion the seemingly complex trick of adding regeneration may be played upon the Equamatic, any t. r. f. set, or any neutrodyne. Such a trick should double the stations one can hear on a good night — at least, it did so in the Laboratory. Mr. King's trick consists in adding regeneration to the detector of the circuit by means of the primary of the r. f. transformer which connects the previous tube to the detector. The only additional instrument necessary is a small variable condenser. In some cases a r. f. choke may also be necessary. — The Editor fig. 1 The conventional detector circuit preceded by the primary of a transformer which couples this detector to a preceding radio-frequency amplifier as well as one across the audio input, C2. Condenser, Ci will be about 1 mfd., effectively conducting r.f. currents through the primary coil back to the filament of the last r.f. amplifier tube and keeping them from running all over the plate in the B-battery leads. Condenser C2 is usually about .001 mfd. and is placed across the audio amplifier input so that r.f. currents in the plate circuit of the detector are given an easy path across the amplifier. A better way to connect C2 is from the plate to the filament of this tube so that these r.f. currents need not go through the B battery or plate supply. Of course, there will be a tuning condenser ■inuunuiiiiiiiiiiiuiikmiiiiij and some other apparatus, but since we shall not need to discuss them further we shall dismiss them from the argument at once. Let us revamp this diagram so that it looks like Fig. 2, which from the viewpoint of radiofrequency currents differs not at all from Fig. [. Condenser Q. connects the lower ends of both primary and secondary coils, and C2 goes from plate to filament. Now let us look at Fig. 3, which is a simple regenerative detector. We have added coil T in series with the audio amplifier and the plate of the tube. All radio-frequency currents in the plate circuit of the detector must go through this coil before returning to the filament of the tube. These changes are small, but the difference such a slight circuit change makes to weak signals is remarkable. Regeneration in a detector 210