Radio Broadcast (May 1928-Apr 1929)

282 RADIO BROADCAST SEPTEMBER, 1928 FIG. 3 and 15" long. The primaries of the r.f. transformers consist of 50 turns of No. 38 enamelled wire on a piece of 1 \" diameter tubing slipped inside of the secondary form. Two of these tuned circuits feed the two remaining screen-grid amplifier tubes, S5 and S6, while the third circuit feeds the detector tube, S9. The actual measured voltage amplification of one individual stage is shown in curve C, Fig. 1 , and is seen to vary from 14 per stage at 550 meters (545 kc.) to 25 at 200 meters (1500 kc). While this amplification may seem very low for a screen-grid r.f. amplifier stage, it must be borne in mind that the high amplification factor of screen-grid tubes has no direct bearing upon the actual amplification that may be obtained from them in practice; that the maximum voltage gain which can be had from these tubes in the broadcast band with practical circuits will vary between 30 and 65 per stage, but that in order to obtain such amplification selectivity must be thrown to the winds. In the Screen-Grid Six, this has purposely not been done and the effective amplification of the three shielded r.f. amplifier stages has been purposely held at a low value in order that maximum possible selectivity could be obtained in these circuits. The overall voltage amplification of the three screen-grid stages, neglecting the antenna coupler, is shown by curve D of Fig. 2, and will be seen to vary from 2500 times at 550 meters to 15,500 times at 200 meters (curve D is simply the cube of curve C of Fig. 1). An important thing to note at this point is the relative selectivity of the three-stage r.f. amplifier unaided by the antenna circuit (which contributes a very high degree of selectivity in itself). The selectivity curves in Fig. 2 do not show that the receiver is, like all t.r.f. sets, in 8 WIRE CABLE BP 2 BP1 / BP3 III H herently non-selective at the lower frequencies. In curve H, Fig. 3, is shown the calculated r.f. amplification from antenna to detector grid for the Screen-Grid Six. (This amplification was calculated because of the practically impossible task of measuring the overall gain of such a sensitive receiver.) The curve is based upon the actual measured amplification for individual amplifier stages, and corresponds to curve D multiplied by curve A. Inasmuch as regeneration is not seriously present on the longer wavelengths (low frequencies) there is every reason to believe that the curve represents the actual performance of the receiver above 300 meters (1000 kc). The rapid rise in amplification of the calculated curve below 300 meters is offset by the fact that on these lower waves there is a tendency for the receiver to oscillate, which is in turn offset by reducing the potential on the screen grids of the r.f. amplifier tubes by adjusting the potentiometer, Rj. The effect of this reduction is to increase the plate impedance of the r.f. tubes, which, in turn, decreases the effective amplification and increases the effective selectivity. The net result is a flattening off of the overall amplification curve much as shown by the dotted lines of Curve I, Fig 3, (curve I represents the actual performance of the receiver). It is seen to be quite flat, though the individual curves composing it were anything but flat to start with. Before passing on from the r.f. amplifier, it is well to mention that every precaution has been taken to render the performance of this portion of the receiver as stable and dependable as possible. This can easily be realized from an examination, of the illustrations and diagrams, which reveal individual copper stage shielding for the tuned r.f. amplifier circuits, individual bypassing of all B-supply leads by condensers directly in the stage shields, and the isolation of all r.f. currents from any common paths which might cause coupling and instability. The antenna input circuit is thoroughly shielded from the three remaining r. f. circuits, and when the receiver cabinet is in place, it is thoroughly shielded from extraneous interference. In order to allow for compensation of varying antenna characteristics, the option of two methods of antenna coupling is provided. One method employs a variable selectivity control in the form of a 75-mmfd. antenna series condenser, C5. In 30 50 70 100 200 300 500 1000FREQUENCY-C.P.S. 2000 5000 10.000 CI C2 C3 C'5 FIG. 5. ARRANGEMENT OF PARTS ABOVE SUB-PANEL FIG. 4 the other method the condenser is omitted, and the antenna taps the primary coil, Li, directly. The single tuning condenser, Q, tunes the antenna circuit, and the triple gang condenser, C2, C3, and C4, tunes the three remaining r.f. circuits housed in shields SHi, SH2, SH3. Three compensators on the condenser frame allow compensation, once the set is assembled, for variations in tube and circuit capacities. Oscillation over the lower portion of the broadcast band, and volume over the entire band, is controlled by the potentiometer, Rlf which varies the potential on the screen grids of the r.f. amplifier tubes, S4, S5, S6. The detector, S9, presents no unusual features, being the conventional gridcondenser, C13, and leak, R7, type with negative filament return, since this was found to give best results in the Screen-Grid Six. THE AUDIO CHANNEL T^HE transformers employed in the a.f. amplifier, Ti and T2, will be seen to consist of auto-transformers, resonating condensers, and plate resistors, all sealed in individual cans. These transformers have an effective transformation ratio of about 4.3 for Ti, and 3.5 for T2, and through a unique phenomena of resonance obtained from proper proportioning of the auto-transformer windings, the condenser and the resistance, together with the plate resistance of the tubes used, a rising low-frequency characteristic is obtained which provides a hump in the amplification curve just below 100 cycles. (Fig. 4) A description of the audio amplifier system will be found in an article by Kendall Clough in the July issue of Radio Broadcast (pp. 133-4) The effective voltage amplification of transformers Ti and T2 with a CX-301A or CX-112A first stage tube and cx-i 12A or cx-310 output tube would be approximately 960, a value much higher than is obtained from an ordinary transformer amplifier employing 3:1 transformers which would give a voltage gain of only 575 times as well as a bad fall-off in low-frequency amplification. (For the benefit of the dyed-inthe-wool fan who may think to improve ordinary transformers by either choke or resistance parallel feed, let it be stated that this canL4 not be done by rule of thumb methods — the Clough system has to be carefully proportioned mathematically to attain the results shown in Fig. 4.) • ' The photographs, drawings, and parts list are clearly marked and keyed and require practically no explanation. Mechanically, the receiver consists of a pierced metal chassis 2iTV long, a} |" wide, and I" deep. On top of the chassis are fastened, at the left end, the antenna coil, Li, the antenna tuning condenser, Q, as well as antenna and ground binding posts BPi, BP2, CI2