Radio today (Apr-Dec 1939)

Record Details:

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Fig. 3 — Typical separate oscillator and mixer circuits. Fixed bias for 6L7G modulator grid is developed across 100M resistor. common size. Intermittent oscillations may be traced to a grid condenser of too bigb capacity. The reason for a large capacity stopping oscillations, is that the dc grid bias is held at too constant a negative value. Sudden load variations which make the plate current drop, do not permit the gridbias to reach zero fast enough to "pulse" the plate current and maintain the oscillation. The plate or screen by-passes in an electron-coupled oscillator are the paths through which the rf currents must return to the cathode. Their reactance, or opposition to the flow of these currents must be small. A value of 0.01 mfd. is generally the smallest value used. At 1,000 KC, a 0.01 mfd. capacitor has a reactance of 16 ohms. The oscillator grid bias is developed by the grid rectified rf current flowing through the leak. Triodes used in most supers employ a 30,000 to 50,000 ohm resistor as a grid-leak. This value is not extremely critical and may vary 5,000 ohms without a noticeable effect. Fig. A — Pentagrid mixer type circuit with Hartley oscillator connections. GRID BIAS CHECKS OSCILLATIONS The oscillator may be checked for operation by measuring the grid voltage. A vacuum tube vm that has a low reactance test-lead and does not "pull" the circuit out of oscillation, is necessary for direct observation. If such a meter is not at hand, low-range milliammeters may be placed in the cathode and plate circuits (for a triode) and the cathode current minus the plate current is equal to the grid current. The product of the leak resistance and grid current gives the grid-bias. A 500 microammeter may be placed directly in the grid lead to indicate the presence of oscillation and bias voltage. The stability of the oscillator over the band may be observed with the vtvm or the ammeters. The grid current and bias should remain quite constant or have a uniform rise toward the low frequency end of the band. The plate or screen current will be minimum when the oscillator is working and will vary in the opposite way from the grid current. Intermittent operation may be checked in oscillator circuits by watching grid current or bias. Two other requirements of the oscillator are proper magnitude of voltage delivered at the mixer stage and proper frequency of this voltage. LOWER HISS WITH HIGH OSCILLATOR VOLTAGE Since the strength of the IF signal is directly proportional to the oscillator voltage, it is desirable to have as high an oscillator output as possible. While converter stages have a higher "hiss" noise than pentode amplifiers, the "hiss" level is practically independent of the oscillator excitation. Since low conversion gain accompanies low oscillator excitation to the mixer, an abnormally high noise to signal ratio may exist. Therefore, the "hiss" noiselevel may be reduced by increasing the signal voltage delivered by the oscillator. The lower curve shown in Fig. 2 gives the relation between noise-to-signal ratio and the root-mean-square voltage delivered by the oscillator to the mixer grid of a 6L7. It is noticed that beyond 10 volts RMS. the noise-signal ration is practically fiat and at a minimum. (The root-mean-square value of an AC wave is the square root of the average of the squares of all the instantaneous current or voltage values over a cycle. If one RMS ampere were passed through a resistance, it would dissipate the same amount of power as one ampere of DC.) The conversion conductance of a mixer is the ratio of the IF signal current developed to the rf signal voltage producing it as the value of the signal voltage approaches zero. (Conductance is the ease with which a current flows in a resistive circuit and is the reciprocal of resistance.) The value of conversion conductance for a well designed stage is between 300 and 600 micromhos, depending upon the type of tube used, and the electrode voltages used. COMBINATION MIXERS USED The coupling of oscillator voltage to the mixer tube is usually capacitive. In the Majestic model shown in Fig 3, the oscillator voltage appears between the modulator grid No. 3 and ground. The 100 M resistor for grid No. 3 con Fig. 5— Signal frequency and oscillator frequency must bear a fixed difference to each other. Series padder connection is used to give proper relation between frequency and capacity. nected to the cathode prevents the bias developed by the rectified oscillator voltage from affecting the control grid No. 1 bias. The rectified current flowing on positive oscillator cycles would also flow through the cathode bias-resistor were the 100 M resistor connected to ground. Since it is practically impossible to obtain a uniform oscillator output over the entire range of frequencies, the mixer tube would operate under various efficiencies as the control grid-bias varied. The voltage may be taken from the oscillator grid or cathode in the electron-coupled type of oscillator, and from the plate, or grid, of the tickler-feedback circuit. The majority of superhets today are employing the pentagrid mixer-oscillator tube. This type of tube uses electronstream modulation by both the grid and plate of the oscillator portion. The "plate" of the oscillator section is usually the screen or No. 2 grid of the tube. The oscillator circuit may be a Hartley as shown in Fig. 4 for the General Electric model H600. A more common type is the tickler-feedback circuit similar to the one used in the oscillator of Fig 3. The combination oscillator-modulator stage may be checked for operation exactly the same as the separate stages. CONTROL GRID-BIAS REGULATES GAIN Control grid bias for both pentagrid combination stages, and separate firstdetector oscillator circuits is in the neighborhood of — 3 to — 6 volts. Greater negative bias cuts the conversion gain. To keep the oscillator frequency at a definite relation to the signal fre(Continued on page 49) NOVEMBER, 7939