UX222 AS SHORT-WAVE R.F. AMPLIFIER

Radio Record, Volume I, Issue 35, 16 March 1928, Page 11

UX222 AS SHORT-WAVE R.F. AMPLIFIER

The following diagram of a_shortwave receiver, employing the UX222 as R.F. amplifier, is taken from the December number of "Q.S.T." | The set is built in three separate aluminium or copper boxes, each 5 by 6 by 9 inches. One box holds the R.F. stage, another the detector, and the third the Q-stage amplifier, which is not shown in the diagram. Another smethod is to build one large box with partitions. As plug-in coils are used, the top of the boxes must be easily removed. Many experimenters will be satisfied to try an R.F. unit alone, which may: be made up in a small copper box.

> C-.006 mfd. C1-.0001 mfd. C2-.1 mfd. C3-Variable, .000075 (?). Ri-10-ohm fixed resister. R2-12 ohm rheostat. R3-50,000 ohm variable resistance. It must be distinctly understood that this circuit has not been personally tested, but from its origin may be cousidered quite reliable, and is given here for the benefit of hny reader who wishes to do his own experimenting. When the R.F. unit alone is con. structed it must be particularly noted that some slight alterations will be necessary in the wiring of the present detector circuit, but these will not imterfere with its normal operation. It will be necessary to connect the grid leak from the grid to the filament of the valve, instead of across the grid condenser. A .002 mfd. fixed condenser must be connected between the tuning-coil and condenser at the earthed end, the condenser being left connected to earth. A lead from the earth side of the coil goes to the B battery to supply voltage to the plate of the R.F. tube, and another from ithe grid end of the coil goes to the ‘plate of the amplifier. When the R.F. valve is not being used the lead to the B battery will cause no trouble, and the lead from the grid end of the coil may go to the antenna through a small capacity, or may be left disconnected, and an aerial coupling coil used if this is desirable. As a volume control, a 500,000-ohm variable resistance is recommended to be shunted across the secondary of the last amplifying transformer. Because the tube is to be operated as an amplifier, it is necessary to apply a biasing voltage of from 1 to 1.5 volts to the control grid when using from 90 to 185 volts on the plate. ‘The outer or shield grid is connected to the plate battery at about 45 volts, although this is not at all critical. A by-pass

condenser (*) ensures a low impedance radio-frequency path from the shield grid to the filament and earth. The’ extremely high impedance of the valve is most easily matched by use of a tuned trap circuit, and the voltage across it is applied to the guide of the succeeding valve. In order to do away with dry cells for obtaining C voltage, this is obtained as a drop off the filament resister. | | "PREE"" GRID-BIAS. If a 10-ohm fixed resister is connect: ed in series with a 12-ohm rheostat, the voltage drop across the fixed unit, when the proper filament current is flowing, will be 1.82 volts, a value that is satisfactory for bias purposes. As the bias on the grid must be negative, the resister must be connected in the negative filament lead, and in order to impress only 1.32 volts on the grid, the unit should be connected directly to the filament. The positive A battery is connected to earth, and to the copper shield, as are the rotor plates of the tuning condenser. It is, therefore, necessary to apply the bias voltage between the rotor of the tuning condenser and the low potential, or earthed end, of the tuning inductance. The use of a pair of by-pass condensers takes care of the radio-frequency currents that flow in the grid circuit. SEPARATE DETECTOR B BATTERY. A separate B battery is used for the detector, but when it is considered that this also prevents feedback between the detector and R.I*. circuits, which might easily be caused by the internal tesistance of the B battery, it is well worth having. A ‘small-sized dry B battery will be quite suitable. A fixed condenser of 1 mfd. should be placed across the main B battery, and in the plate lead from this battery to the audio amplifiers, a radio choke should be inserted to prevent the bvypassing of R.F. energv from the radio stage through the andio circuit. The inductances are wound on bakelite tubes or other form,,and may plug into a strip mounted directly on the tuning condenser. The tickler and aerial coils are bunchwound affairs supported by their own leads. ‘They are slightly over an inch in diameter, which tends to reduce the amount of reaction caused by the receneration control on the tuning. Regeneration and oscillation are controlled by a variable resistance, which is usually not by any means as satisfactory as condenser control. Tf a sufficient number of readers are interested in the circuit, further details and diagrams will he given.