Improving Low-Frequency Response

Author: N.H. Crowhurst

Now that we know the action of the basic components, we can see what will happen by changing their values. First, consider the effect of changing the plate coupling resistor. In the case of a triode tube, the plate resistance of the tube is between plate and ground, while the plate coupling resistor is between plate and B+. The B+ supply will invariably have a very large capacitor coupling the high voltage to ground, so it is impossible for the B + voltage to vary relative to ground. Any audio currents that reach the B + point will be bypassed to ground through the large output capacitance in the B+ supply.

Points of a-c ground or zero potential

In the case of a triode tube, the coupling resistor is usually larger than the plate resistance of the tube because the a-c resistance of a triode is much lower than its d-c resistance. The plate coupling resistor will be about equal to the d-c resistance of the tube, so that about half the B+ voltage drops across the coupling resistor and half across the tube.

Improving the low-frequency response of an amplifier

Improving the low-frequency response requires the use of either a larger coupling capacitor or larger resistances in the associated circuit. When the preceding tube is a triode, increasing the coupling resistor will not effectively increase the resistance for audio voltages from the plate to ground because this resistance is limited by the plate resistance of the tube. The only possibility of considerably increasing the resistance associated with the coupling capacitor is to increase the following stage grid resistor. Alternatively, the coupling capacitor can be increased in value.

Improving the high-frequency response

For high-frequency signals, the coupling capacitor is effectively a short circuit - no audio voltages appear across it, and both sides of the capacitor are at the same audio voltage at any instant. This means that we now have three effective resistance paths to ground from the plate of the tube: through the plate resistance of the tube, through the coupling resistor, and through the following stage's grid resistor.

Improving high-frequency response can be achieved by reducing the small capacitances to ground or reducing the total circuit resistance to ground. In the case of a triode amplifier, the lowest individual resistance (which principally controls the effective parallel resistance of the combination) is the plate resistance of the tube, which is fixed by the tube type. Consequently the best way to approach getting better high-frequency response is to see how we can reduce the capacitance.

One way to reduce capacitance is to use a smaller coupling capacitor (smaller in actual dimensions, rather than smaller in value). The coupling capacitor, like all other parts of the audio wiring, will add to the capacitance to ground, in addition to providing the requisite capacitance between its plates. The only way to reduce its capacitance to ground is to use a capacitor of smaller dimensions, so that it does not have such a large surface to provide capacitance to ground.