Positive feedback can only be used over one stage in order to avoid oscil-
lation. An easy way to accomplish this positive feedback is to couple cathode
bias resistors of two consecutive stages in the earlier part of the amplifier,
where the distortion is small.
A momentary positive fluctuation at the grid of the first stage will pro-
duce a momentary negative fluctuation at the plate, which is passed on to
the grid of the following stage. This produces another positive fluctuation
at the plate of the second tube. At the same time, the negative fluctuation
at the plate of the first tube, resulting from increased plate current, will be
accompanied by a positive fluctuation at its cathode. Similarly, a negative
fluctuation appears at the cathode of the second tube. The fluctuation at
the cathode of the second tube is much bigger than that at the first stage.
Connecting the resistor between the cathodes will allow some of the fluctua-
tion from the cathode of the second stage to cancel the fluctuation at the
cathode of the first stage.