Improving the Frequency Response of a Pentode Stage

Author: N.H. Crowhurst

Using a pentode tube for the stage before the coupling alters the situation. Here the plate resistance is usually much larger than the load resistance, because the a-c resistance of a pentode is much greater than its d-c resistance. This means that the load resistance used with a pentode exercises the principal control on the plate-to-ground resistance. Using a larger coupling resistor, as well as increasing gain (which it naturally does with a pentode), will improve the low-frequency response, because it increases the resistance in series with the coupling capacitor. At the same time, the increased load resistance will reduce the high-frequency response because it makes the total resistance to ground higher.

Frequency characteristics of a pentode stage

Thus whichever type of tube we use, there are optimum (or best) combinations of circuit values to get the kind of response we want: a response that is good enough, but not unnecessarily good. In each case, struggling too hard to get a good low-frequency response will give us problems at the high-frequency end, and vice versa. Further than this, if we struggle to get a good frequency response at both ends we shall get less gain from the amplifier. The values that we use will be such that the first tube cannot develop as much gain as it would if we were content with a frequency response that was not quite so uniform.