An idle current of 70 mA requires a 10 ohm sense resistor, as the transistor's base-to-emitter voltage is 0.7 volts. The math is easy:
  R = base-to-emitter voltage / idle current.
Fortunately, the principle of operation is also easy. If the output tube's idle current exceeds 100 mA, the voltage developed across the 10 ohm resistor will exceed 0.7 volts, which will cause the transistor to increase its conduction, which in turn will pull down the tube's cathode voltage, which will effectively make the grid more positive to the cathode, which will cause the tube to conduct more current. This increase in current will develop a greater voltage across the plate resistor and push down the driver tube's voltage. As the output tube's grid is directly attached to the driver tube's plate, the output tube's grid will also be made more negative relative to its own cathode, which will lessen its conduction until the voltage across the 10 ohm resistor returns to 0.7 volts.
   On the other hand, if the output tube's idle current drops below 100 mA, the voltage developed across the 10 ohm resistor will fall below 0.7 volts, causing the transistor to decrease its conduction. This decrease in current will develop a lesser voltage across the plate resistor and pull up the driver tube's voltage and the output tube's grid will also be made more positive relative to its cathode, increasing its conduction until the voltage across the 10 ohm resistor returns to 0.7 volts. This is how a DC servo loop works: the circuit strives to maintain a target voltage and makes adjustments to its output voltage or current until that voltage is achieved. (An added twist might be to add a second transistor and break the 10  ohm resistor into two resistors: 3 and 7 ohms. This second transistor would serve to prevent the output tube from being damaged when the input tube is removed while the amplifier is in use. By connecting the base of this transistor to the 7 ohm resistor and connecting its collector to the grid of the output tube, we have built in idle current limit of 100 mA.)

Lofton White output stage with DC feedback
and a failsafe current limiting circuit

DC Servos and Class A Operation
  The key point to remember is that we are dealing with the idle current. In a Class A SE amplifier, the idle current and the average current flow during use are the same. This is not the case in a Class B, AB, or C push-pull amplifier. But it is the case in a Class A push-pull amplifier. In a Class B amplifier, the average current flow during use can be ten-fold the idle value.
   Unless something like a voltage clipping circuit is used to prevent the servo circuit from being tripped up by the signal being reproduced, the servo circuit will not work on any amplifier other than a strict Class A type. If the amplifier is not truly Class A, then the circuit will cause great distortion as it tries to turn off the output stage by dropping the bias voltage in an attempt to force the average current to equal the idle current. This explains why the auto-biasing circuit outlined by Brian Lowe in Glass Audio (0/88. page 9) does not work. This also explains the inability of some supposed Class A amplifiers to work with a DC servo for setting the idle current.
   Fortunately, most simple tube circuits are run in Class A; for example, if the circuit is single-ended (Cathode Followers, Grounded Cathode amplifiers), it must work in Class A.

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