Copyright ©1996-2004 GlassWare        All rights Reserved.

Ground-Cathode Transformer (cathode bias) Amplifier

This is but a small variation on the classic push-pull transformer-coupled tube amplifier topology, which uses a common cathode resistor to set the idle current of the output tubes. This topology works best when the output stage is run in class-A or heavy class-AB, but in no way does a cathode resistor strictly imply class-A operation. Fixed biased amplifiers can be biased to rich class-A and amplifiers that share a common cathode resistor can be biased to a lean class-AB. A pure class-B, wherein all the output tubes are completely cut off at idle, is impossible, as some current must flow in order to define a voltage drop across the shared resistor.

The cathode resistor should be bypassed with a large-valued capacitor to prevent the bias voltage from varying with the input signal. In a lean class-AB or class-B amplifier, the cathode resistor could be replaced with a zener diode, which should also be bypassed with a large-valued capacitor. (If the idle current is high enough, for example 150mA, the cathode resistor can be replaced with an input tube's heater element, making for a free DC heater power supply for the input tube.) On the other hand, in a pure class-A amplifier, the common cathode resistor can be left unbypassed or even replaced by a constant current source. In all cases, power supply voltage will be lost in biasing through the cathode; thus, less power will be available from this configuration than from a comparable fixed-biased amplifier. So why bother with cathode biasing?

Sometimes the answer is simply that the power transformer did not come with a bias voltage tap. A better answer is that a self-biased amplifier is less likely to suffer from bias runaway, wherein an output tube conducts so much current that the tube or output transformer is damaged. Self-biasing is a form of DC negative feedback, as any greater current conduction will define a greater voltage drop across the common cathode resistor, resulting in a greater negative bias on the tube's grid, further reducing its conduction. Furthermore, self-biased amplifiers often seem to clip more gently than their fixed-biased brethren. Since all amplifiers clip, softer clipping can avoid the early onset of listener fatigue.

More Information On Schematics