Radio Broadcast (Nov 1926-Apr 1927)

MARCH, 1927 AN AMPLIFIER FOR THE R. B. "LAB" RECEIVER 469 FIG. 4 that, due to the close placement of the transformers and sockets, exceedingly short leads between these parts are possible. It is possible to run much of the wiring, especially the longer leads, under the base-board, through holes drilled near the various terminals. When the wiring has been completed, a box may be assembled around the base. A light coating of shellac will fill up the pores of the wood after which a coat of aluminum paint may be applied to the entire cabinet sur potential when a certain fixed value of grid bias is applied. In the case of a C battery detector there are certain values of B battery that must be employed in conjunction with the value of the grid bias used. Both go hand in hand. A table of B and C voltages for various output tubes is given here. ux 1 12 ux 171 c B c B 6 90 i6| 90 9 135 27 135 1575 40* 180 Under all normal conditions the C battery detector will work satisfactorily with 4.5 volts on the grid and 45 on the plate. Some tubes, however, refuse to oscillate over the whole band with these voltages. The remedy is obvious; increase the B voltage to 672 or increase the regeneration capacity by twisting a pair of insulated wires and If it is desired to employ the new detector tube, ux-200-A, it is well to replace the grid leak and condenser to the detector circuit and to run the grid return to the negative filament lead. Stabilization of the receiver, together with more accurate neutralization, is bound to occur when the bypass condensers are applied to the radio-frequency amplifier and detector circuits. This is especially true of the C-battery circuits because in most cases the leads to the C batteries are quite long with every chance for serious coupling or pickup. The bypass condensers eliminate the probability of the coupling and in turn make the receiver more possible of complete neutralization. It is not amiss to include the bypass condensers, together with the necessary r. f. choke, in the plate circuit of the r. f. tube so as to provide complete bypaths for the r. f. currents. This precludes the possibility of r. f. currents finding their way into the battery circuits. The circuit diagram, Fig. 6, shows the addition of bypass condensers, (Cx) and r. f. choke coil (Lx) to the radio-frequency amplifier and detector circuits. THE EXTERIOR OF THE AMPLIFIER CABINET For the fastidious constructor who desires the same color scheme for his amplifier as his two-tube R. B. "Lab" receiver, (which is finished in natural aluminum), a box can be assembled to fit over the amplifier, and be finished with aluminum paint. The illustration shows such a box with the amplifier inside l-Top 8H" wide »14%" long " %" " FIG. 5 face. This latter coat is only necessary when the builder desires to have his amplifier look like the aluminum cabinet of the two-tube shielded R. B. "Lab" Receiver described in the January Radio Broadcast. Fig. 5 shows the assembly details for the box, together with a complete material list of the wood required. The manner in which the various pieces are assembled is clearly shown, and no difficulty will be experienced in this respect, providing the constructor has taken care to cut his wood exactly as specified. The box fits over the base upon which is mounted all the amplifier apparatus, and in the front is cut an opening so as to allow access to the connector plug, and jacks, etc. After assembling the box, carefully sandpaper the surface. The finer the surface, the better will be the finish of the aluminum paint. If brads are used to hold the pieces together, they should be countersunk, and the small head holes filled with soap. If screws are used, be sure to spot the holes first, then drill out, with a drill several sizes smaller than the diameter of the screw. CORRECT PLATE POTENTIALS VALUES of batteries for use with receivers and amplifiers depend chiefly upon the type of tubes employed. The output tube of an audio amplifier, if it be of the semi-power variety, necessitates the use of a definite value of plate attach one end of each wire to the regeneration condenser. The other two ends should be insulated from each other. The main advantage in varying the detector operating voltages is in obtaining a state of regeneration which is considered satisfactory for operation over the entire wavelength range. However, it will usually be found that 4.5 volts of C battery with 45 volts plate battery on the detector tube will be universally satisfactory. In the four-tube receiver the variable resistance controlling volume is situated near the left end of the panel while the interstage coil to which it is connected is quite a distance away from it, necessitating long leads which may give rise to intercoupling. By the use of the choke Lx, the r. f. currents are kept out of the resistance and bypassed to the negative line throughthe condenser Cx. 6 6 6 ■4% -3 A + +B C 90 V. FIG. 6