
247
MODULATING
A.F
INPUT
^
GRID
BIAS
±C
Fig. 15.3 Outline of Heising method of amplitude modulation
It used to be common practice to use a single inductor (a 'choke
coil') in place of the transformer, and to connect the final a.f.
amplifying valve across C, so that it was fed from the same source as
the r.f. valve. This form of anode modulation was known as choke
control.
Because of the large amount of power and heavy equipment
needed for anode modulation, the a.f. voltage was sometimes
applied to the grid. This was not so simple or satisfactory as might
have been expected; it tended to increase distortion and certainly
reduced the r.f. efficiency ...