Grid Voltage Auto-Biasing
Our circuit must monitor the current flow through the tube and make adjustments to the DC component of the grid voltage to compensate for any deviation in idle current. The easiest way to read the current flow is to place a resistor in series with the cathode and measure the voltage that develops across it. Since we do not need more than a volt or so to make an accurate measurement, the cathode resistor can be as little as 10 ohms and ½ watt in rating. Once we have our sampled voltage, it must be compare to a reference voltage. Here is where the precision voltage reference, the LM385, comes in handy. This device is a small 2 lead IC that is much more accurate and quiet than any zener diode. Furthermore, it can operate with a much

smaller current flow than the zener diode: from as little as 20 µA to 10 mA. Its nominal reference voltage is 1.24 volts.
The role of operational-amplifier is to compare the two voltages and adjusts the grid voltage to compensate. The op-amp is ideally suited to perform this task, as it uses all of its available feedback to eliminate a difference in voltage between its positive and negative inputs. It stops adjusting the grid voltage when its senses no difference between its inputs. The trick is to make sure it can accurately read the voltages. In the circuit above, no trick is needed. The op-amp is configured in inverting mode and it samples the voltage across the 112.4 ohm resistor via the 1 meg resistor. The high frequency component (those frequencies above 2 Hz) voltage is shunted away by the .1 µF capacitor. The

reference 1.24 volts is fed directly into the positive input of the op-amplifier. The simplicity is complicated by the need of a positive power supply connection for the op-amp and by the fact that op-amps that can withstand 65-120 volts are both rare and expensive  ($20 per). Of course, if a positive power supply connection for the op-amp is available and you have the $80 (stereo and push-pull amplifiers) to spare, this circuit is the only way to go.
An alternative circuit that will not hurt your pocket is shown at the left. It is functionally identical to the previous circuit, but it has been configured to work with only a negative power supply and to use a much less expensive IC ($1).

The first problem we face when we limit the op-amplifier to a negative only power supply is that the input can only work with input voltages that are no greater than its power supply connections. (There certain FET or MOSFET input IC's that might work with an input of 1.24 volts higher than its positive supply voltage. If you know of one, please send us an e-mail.) This means that we can  no longer directly read the voltage on top of the cathode