Why isn't a forward biased diode not a diode detector?

Frank Dresser

It can be a detector.
The germanium diodes can have a tiny current to keep about
0.25 volts across them even with no signal.
the presence of a carrier with modulation or no modulation will cause a ripple
voltage into a cap, just like the signal at a power supply rectifier.

The ripple voltage is created by a small % of the 455kHz signal cycle
charging the C1 of the filter.

The amplitude of the carrier voltage varies at a slow speed of audio, and the
ripple voltage
stays the same value, and the detector audio signal closely follows the
shape of the modulation, ie, the audio is recovered linearly.

If you don't have any idle current in the diode, and drive the diode
off the end of a grounded IFT coil, then the ripple voltage varies a lot at low
signal,
when the R discharging the C1 of the filter has very little voltage across it.
So low level signals are very distorted by cut off distortion on the audio
cycles.

Patrick Turner.