Overview
Last month we covered auto bias schemes that moved the cathode voltage to set the desired idle current; this month we will examine adjusting the DC component of the grid voltage, instead. A quick reread of April's circuit of the month's overview and design goals will serve to remind us of the purpose and limitations of auto bias circuits. The advantage of grid bias adjustment lies in the elimination of the cathode resistor bypass capacitor and the preservation of B+ voltage.

When a tube is cathode biased, the cathode resistor's value is multiplied by the mu of the tube plus one and then this effective value for the resistor is added to the plate resistance of the tube. This increase in the plate resistance and hence the output impedance of the circuit may not be a concern in a small voltage circuit, such as a preamp or driver stage, but in a power amplifier the damping factor will suffer substantially. Consequently, the cathode resistor is usually bypassed by a large valued capacitor. The value of the capacitor is chosen by picking a low frequency cutoff point, at which the capacitor's reactance will equal the resistance of the cathode resistor. For example if 20 Hz sets the lower limit in frequency response and the cathode resistor equals 800 ohms, then the value of the capacitor would be µF. This capacitor is very much in the signal path, as all the AC

fluctuations in current as the amplifier plays must travel through this capacitor. Better quality capacitors certainly help, but, contrary to advertising copy, no capacitor is good as no capacitor. Grid bias eliminates the need for this capacitor, as the cathode resistor is no longer needed to establish an elevated cathode voltage, but to afford a means of monitoring the current flow through the tube by developing a small voltage. A small voltage means a small resistor value, which means that its ability to soften the output impedance is also greatly decreased; thus the need to bypass this resistor is removed.

In addition, because the cathode resistor is no longer needed to establish an elevated cathode voltage, the voltage that had been developed across it can now be developed across the tube. In a 300B based circuit this might mean a gain of 80 volts of effective B+ voltage. (Of course there are situations where the loss of voltage across the cathode resistors is welcome because of too much B+ voltage for instance.)

A grid voltage auto-bias circuit mimics the way we would adjust the bias on an amplifier: first we would measure the voltage across the small valued cathode resistor (10-20 ohms, usually). Second, we would compare the measured voltage against a predetermined value. Third, if the voltage falls short, we would turn the bias potentiometer clockwise to move the DC voltage on the grid more positive to increase the idle current; otherwise, counter-clockwise to move the DC voltage on the grid more negative to decrease the idle current.

Design Goals
Simple and cheap is what we want from an auto-bias circuit. If we had to give up one goal, then let it be low cost. The more complex the circuit, the greater the chance of a wiring mistake in construction or failure in use. When good tubes cost up to $400 each, the loss of a $3 IC is not what we fear most. In fact, if the auto-bias circuit works well, it should help to prevent any catastrophic damage to the tube.