Cathode Bias - Uses

Uses

Cathode bias is also used to achieve phase inversion. In a more simple circuit, which has less than unity gain, the cathode and plate resistors are made equal. In accordance with Kirchhoff's law, the current through both resistors will be equal, thus the voltage across them will also be equal. As the plate becomes more negative, the cathode will become more positive, and conversely. The resulting signals are capacitively coupled to any succeeding stages, providing a pair of signals 180° out of phase.

The other technique is to use a pair of amplifier tubes with a common cathode resistor. In this case, the input tube is operated as a standard common cathode amplifier, while its twin is operated in common grid mode. The input signal is amplified by the input tube in the normal fashion. An unbypassed cathode resistor, common to both tubes, couples the signal to the cathode of the second amplifier, which is operated in "Grounded grid" mode, with the grid resistor bypassed by a capacitor which maintains a constant grid voltage. The pair of tubes produce outputs exactly out of phase, but the gain of the grounded grid amplifier is slightly higher, requiring that their plate resistances be different in order to maintain balance.

Mathematically, the gain of the phase inverting stage is given by the product of the amplification factor and the load impedance divided by the sum of the plate resistance and the load impedance. The gain of the in phase stage is given by the product of (one plus the amplification factor) and the load impedance divided by the sum of the plate resistance and the load impedance. In order for the gains to be equal, it is customary to use different values of plate resistance. For example, in such a phase inverting circuit using a 12AX7, the inverting stage would have a plate resistor of 100KΩ while the in phase stage would use a resistor of 82 KΩ. Mathematically, it works out pretty close.

Another problem is a slight reduction in gain. The cathode, or bias resistor appears in series with the plate, or load resistor. The bias voltage must be subtracted from the total "B" or HT voltage in gain calculations. In most circuits, this problem is easily overcome by selecting a load impedance at least two orders of magnitude greater than the bias resistance. For example, a 1K bias resistor will have virtually no effect if the load impedance is at least 100K. These values were, in fact, used by Leo Fender in many of his guitar amplifier designs. Refer to "The Tube Amp Book" by Gerald Weber, schematic section.