Nostalgia Air: Grid Bias Resistor Calculations

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Grid Bias Resistor Calculations

RCA Victor RADIO TUBES

The radio service man often finds it necessary to replace the grid bias resistor in receivers employing a self-biasing arrangement for obtaining the proper grid voltage. When the resistance value is not known, it may be calculated by dividing the grid voltage required at the plate voltage at which the tube is operating, by the plate current in amperes plus the screen current in amperes times the number of tubes passing current through the resistor.
Under the above rule, the grid bias resistor value is given by the following formula:

    E_C1 X 1,000
R = -----------
    (I_B + I_C2)n

where: R= Grid bias resistor in ohms.
E_C1 = The grid bias required in volts.
I_B = The plate current of a single tube in milliamperes.
  I_C2 = The screen-grid current of a single tube in milliamperes.
  n = The number of tubes passing current through the resisitor.

Example:
  It is desired to determine the value of bias resistor used to obtain proper value of grid bias on three type '35 tubes working in the radio frequency stages of a receiver. First determine the plate and screen voltages employed in this set. Suppose, in this case, it is found that the plate supply voltage is 250 and the screen voltage is 90. Looking in the characteristics chart, it is found that the proper grid bias for the '35 under these conditions is -3.0 volts. In addition, the plate current is 6.5 milliamperes and the screen current is 2.5 milliamperes. Substituting in the forumla,

    3.0 x 1,000
R = -----------  = 111 ohms.
    (6.5+2.5)3

  The value of the grid bias resistors can be calculated in this manner for any type and any number of tubes. In the case of triodes, the screen current term drops out entirely.
  Be sure to determine the plate voltage at which the tubes are working, the number of tubes supplied from the bias resistor, the screen voltage, (if a tetrode or pentode), the correct value of grid bias voltage required, and the plate and screen current for the given plate voltage.
  In the case of resistance-coupled amplifiers which employ high resistance in the plate circuit, it must be remembered that the plate voltage is equal to the plate supply voltage minus the voltage drop in the plate load resistance caused by the plate current. The net plate voltage alone determines the correct value of grid bias.
  The foregoing methods of calculations cannot be used in connection with receivers employing a bleeder circuit to obtain grid bias.