Radio Broadcast (Nov 1926-Apr 1927)

Electrifying the aHi*Q" The Construction of an A. C. Operated Power'Supply Device for the Hammarlund' Roberts "Hi-Q" Receiver which Supplies B Current for All Tubes, A and C for Power Tube By LESLIE G. BILES THE Hl'Q IN A CONSOLE Which holds all the power equipment and also a Stevens conoidal loud speaker, the latter being concealed behind the grill IT IS true that, not so many months ago conjunction with a little or no attention was given in the matter of choosing tubes of different characteristics according to the duties they were required to perform in a radio receiver, but nowadays such selection is carefully made by almost every radio constructor. Reference to the circuit diagram of the new Hammarlund-Roberts "Hi-Q" receiver, Fig. I (reproduced from the author's article on this receiver which appeared in the January Radio Broadcast), offers the information that the grid return of the detector tube is connected to the negative filament lead. This connection necessitates the use of one of the new special detector tubes of the 200-A type. For the radio-frequency stages, the 201 -A type tubes can be used. When it comes to the output tube, there seems to be a lot of confusion as to the proper tube to use. Shall it be a 201 -a, a 1 12, or a type 171? The use of a power tube greatly increases the amount of energy that can be delivered to the loud speaker without distortion, but plenty of voltage for the power tube is necessary for full amplification. The relative values of these three tubes as power handling devices when used with the proper grid and plate potentials are as follows: device to obtain some or all of the necessary power for a receiver from the a. c. mains. The following data relates to the construction of a power -supply device that has been designed by the author for use with the "Hi-Q" receiver. It supplies a. c. for lighting the filament of the last tube, B voltage for the entire receiver, and C bias for the last tube. A small-capacity storage battery may be used, in trickle charger and automatic relay, to light the filaments of the other tubes. This combination will result in a set up that will require very little attention except the periodical addition of distilled water to keep the plates of the storage battery covered. A few minor alterations, necessitating the changing of about half a dozen connections of the audio amplifier in the Hammarlund-Roberts "Hi-Q" set, will have to be made so that the power device can be used to light the filament of the 171 tube and supply the high negative bias. The following additional material will be necessary: 1. 10 ■75 .50 .90 .50 .05 .10 1 Samson Output Impedance, Type O 1 Sangamo Series A Condenser, 4-Mfd 1 Pair Carter "Imp" Telephone Jacks 5.00 3.00 .60 1 Tobe Veritas Resistor, 2000-Ohm 1 Durham Metallized Resistor, 50,000 Ohm 1 Lynch Double Resistor Mounting . 1 Tobe Condenser, i-Mfd 1 Yaxley Pilot Light Bracket, No. 310 1 Brass Angle Bracket, \ x § Inches . 1 Piece Panel Material, 3 x 1 inches . Total $11.90 (Similar standard parts may of course be used). The new connections of the amplifier which is shown as part of the circuit in Fig. I, are shown in Fig. 2. To make the changes, it is necessary to follow the subsequent procedure: Loosen the screws holding the audio amplifier to the baseboard. Remove the wire from the 135volt binding post and the wire from the "P" terminal of the output tube. The jack is not to be used again, and may be removed, a pilot light bracket being installed in its place on the panel. Raise the amplifier and remove the wire connecting (at the end of the sub-panel) Amperite No. 3 to the tube socket and the other wire from Amperite No. 3 to the plus A lead of the set. Replace the Amperite mounting with a double resistor mounting. Disconnect the wire running from the minus A binding post to the minus terminals of the audio tube sockets and the soldering lug on shield section 1 D. Connect the minus-A post to the minus terminal of the power tube socket and connect a branch to the lug on the shield. Remove the wire connecting the minus-F terminal of the second audio transformer to the minus-C binding post. Connect the minus-B binding post to both Undistorted Tube Grid Plate Output Volts Volts (Milliwatts) 20 1 -a 4-5 90 >5 1 12 9 135 120 171 40.5 180 700 Since the 171 type, under the conditions given in this table, will handle six times as much as a 112, and forty-seven times as much as a 201-A tube, its superiority is unquestioned. With the advent of the power tube, it has become increasingly economical to utilize a FIG. I