The Grounded-Cathode Amplifier

Determining cathode and plate resistor values

The grounded-cathode amplifier is approaching its centennial (2007) and it remains the building block of most tube audio equipment. It is simplicity itself, with little more than a few resistors and a triode; yet many are ignorant of its inner workings.

A Little History and Theory
   The triode descends from the tube diode (tube rectifier) and as a consequence it shares several features with the diode. For example, it can only conduct current in one direction: from its cathode to its plate (or from cathode to grid when the grid is more positive than the cathode). In addition, it offers some resistance to the flow of that current, which means that like the tube diode, the more current flowing through it, the greater the voltage drop across it. (In fact, if we tie the grid to either the cathode or the plate or if we leave it unconnected, the triode is reduced to a diode.) This resistance is referred to as the triode's rp or plate resistance. (In the old days, rp was reserved for references to the triode's AC impedance; and Rp, the triode's DC resistance. Sadly, today, rp covers both the DC and AC aspects of a triode's behavior.)   
      By allowing the easy control of the current flowing through the triode, the grid makes the triode a useful electronic device. The poor relation, the diode, lacks this feature, as the only way to control the flow current through the vacuum diode--other than to reverse the polarity of the applied voltage--is to vary the voltage between its plate and cathode. In fact, if the diode did not hold the property of unidirectional current conduction, it would be of no more use than a slow-turn resistor, with a relatively short life expectancy and poor linearity. In contrast, by simply varying the voltage on its grid, the triode's current can be completely stopped -- or increased up to complete saturation.

    Mechanical engineers must know the basics of their practice -- lever, wedge, wheel and axle, pulley, and screw -- as these five simple machines form the basis of all other more complex machines. Likewise, vacuum tube circuit designers must know the inner workings of the grounded-cathode amplifier, the cathode follower, and the grounded-grid amplifier, as these three basic circuits form the basis for nearly all complex tube circuits.
     If for some reason we could understand only one of the three basic tube circuits, which circuit should we chose? Undoubtedly, the best choice would be the grounded-cathode amplifier, as it finds the widest use and it embodies the inner workings of a vacuum tube nicely. Yet many do not know how this simple tube circuit works, neither knowing how to select a suitable cathode resistor to set the circuit's idle current, nor how to chose a useful plate resistor (aka anode resistor) value to allow the greatest swing of output voltage. This article offers a simple explanation of the triode and explains how to determine these two resistor values.

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