Radio broadcast .. (1922-30)

THE Roberts receiver, of which the Ham- marlund "Hi-Q" is a semi-commercial model, was first introduced to the radio public by RADIO BROADCAST so many years ago that the author has neither the ambition nor time to go through his files to determine just when Dr. Roberts presented his first article. This momentary reminiscence perhaps has little in common with the point to be discussed in the present writing, but there is significance somewhere in the thought that this is the only circuit, of the many hundreds introduced in broadcasting's nebulous days, that has remained standard and popular to the present time. Simple efficiency is responsible for this consistent popularity. The 1927-1928 Hammarlund Roberts "Hi-Q," described in RADIO BROADCAST for October, 1927, departed somewhat from previous models in mechanical and electrical design, though the ultimate effects are consistently in line with previous designs. The last two models of the "Hi-Q" receiver have incorporated variable coupling between the radio-frequency primary and secondary circuits. The possibilities of such an arrangement were pointed out by Zeh Bouck in an article appearing in the September, 1926, issue of this magazine, entitled "Higher Effi- ciencies in R. F. Amplifiers." The argument, in brief, is as follows: At every frequency or wavelength there exists an optimum value of coupling between primary and secondary circuits—a value of coupling which provides the maximum signal intensity compatible with quality and stability. This optimum degree of coupling varies, however, with the frequency. To maintain optimum con- ditions over the entire tuning range, therefore, it is desirable that the coupling be varied with the wavelength. This is accomplished automati- cally in the Hammarlund-Roberts receiver. The general characteristics of the Hammar- lund "Hi-Q" receiver remain unaltered in the adaptation of this receiver for the use of a. c. tubes, as comparison of the circuits shown in Figs. I and 2 with the direct-current arrange- ment illustrated in the October, 1927, RADIO BROADCAST, will indicate. The changes effected have merely been in the nature of the substitution of heater type a. c. tubes for the d. c. ones, accompanied by slight alterations in the constants of the circuit to compensate changes in tube characteristics. The receiver has been redesigned for the use of two different makes of a. c. tubes, the R. C. A. 227 (Cunningham 327) type and the Arcturus a c. amplifier, detector, and power tubes. The selection of two types of tubes has been sug- gested by motives of general convenience. Electrifying the "Hi-Q' By F. N. Brock THE "HI-Q" WIRED FOR CUNNINGHAM OR R.C.A. A.C. TUBES The use of the R. C. A. tube in the "Hi-Q" receiver will be first considered. The following is a list of the essential parts employed in the construction of the receivers: i Samson "Symphonic" Transformer I Samson Type HW-A3 Transformer (3-1 Ratio) 4 Hammarlund o.ooo5-mfd. Midline Condensers 4 Hammarlund "Hi-Q" Six Auto-Coupled Coils 4 Hammarlund type RFC-S; R. F. Chokes Hammarlund Illuminated Drum Dial Sangamo o.ooo25-Mfd. Mica Fixed Condenser Sangamo o.oooi-Mfd. Mica Fixed Condenser Pair Sangamo Grid Leak Clips Durham Metalized Resistor, 2 Megohms 3 Parvolt o.5-Mfd. Series A Condensers 6 Benjamin No. 9040 Sockets 3 Eby Engraved Binding Posts i Yaxley No. 660 Cable Connector and Cable i Hammarlund Roberts "Hi-Q" Six Foundation Unit (Containing drilled and engraved Westinghouse Bakelite Micarta panel, completely finished yan Doom steel chassis, four complete heavy aluminum shields, extension shafts, screws, cams, rocker arms, wire, nuts, and all special hard- ware required to*complete receiver.) For the construction of or adaptation of an existing "Hi-Q" receiver to one employing the 227 type tube the following additional parts were used in the adaptation: 5 Benjamin Green Top A. C. 5-Prong Sockets i Thordarson Type 2504 Filament Transformer (or Karas AC Former) FIG. I i T2oo Electrad Variable Resistance to Permit Temperature Regulation I o.5-Mfd. "Parvolt" Series A Condenser I 2oo-Ohm "Truvolt" Grid Resistance (Elec- trad) I Samson 3o-Henry Choke or a Samson Type O Output Impedance i 2- or 4-Mfd. Series A "Parvolt" Condenser i Electrad Type J Resistance for Volume Control 5 R. C. A. uv-227 or Cunningham c-327 Tubes i ux-i7i-A or cx-37i-A power tube. CONSTRUCTIONAL DETAILS THE construction of the receiver remains practically identical with that of the direct- current models. The general layout of the parts and the mechanical mountings have been described in detail in articles on the d. c. set and in the Hammarlund Roberts "Hi-Q" Six Manual. The five-prong sockets are mounted in the same places and with the same screws as the old sockets. An extra hole for the cathode lead must, however, be drilled just under the K or cathode terminal In the a. c. models of the Hammar- lund " Hi-Q" the right-hand control (the rheostat in the battery-type receiver) may be used to control a I lo-volt line switch, such as the Carter "Imp" type 115. The similarity of the a. c. and the d. c. me- chanical layouts is evidenced by comparing the accompanying photographs with those of the d. c. models which have frequently appeared. The circuit of the Hammarlund Roberts "Hi-Q" Six receiver employing type 227 tubes is shown in Fig i, in reference to which the fol- lowing points are worthy of mention: All filament or heater wiring should be made with a twisted conductor. It is desirable that consistency be observed in the socket connec- tions with these power leads. In other words, it is preferable that the same heater terminal on each socket be connected to the same heater lead. This is most readily accomplished by em- ploying a twisted pair of two colors. Corwico flexible red and black Braidite is a convenient recommendation. The red and green leads on the Yaxley cable are not used. Heater tubes are employed throughout the circuit (with the exception of the output ampli- fying stage) due to the simplicity and consist- 436