Interstage transformers were very often used between LF
stages in early battery sets. Commonly they were of 3:1 or 5:1 step up
ratio. This voltage magnification was an important source of gain. It
depends on the ratio of the source to driven impedances being
significantly greater than the inverse square of the voltage gain. The
high impedance of the grid circuit of a valve is essential. This
coupling method is the primary reason for the low anode impedances
designed into most battery triodes used as detectors and LF stages
before the output.
