How does an oil can delay work?

Vintage oil can echo devices are electromechanical units that write a
static charge onto a rotating metal can, using oil as a lubricant that
helps retain the charge on the can. The resultant delay signal is very
band limited, creating a murky sounding echo. But that’s not the only
quality that gives the oil can echo its atmospheric magic.

The units have a record head and typically two playback heads, but
generally no erase head, allowing some dissipating charge to remain as
the can rotates. This contributes to two unique characteristics of these
systems:

• First, it creates a regenerating repeat even when no feedback
(REGEN) is applied from the playback heads. The static charge has
its own decay time constant, so there are always “repeats” from the
echo at all settings.

• Second, it creates an uneven cadence to the echoes, where the
first echo occurs based on the distance from the record head to the
playback head. But subsequent echoes occur at the rotational speed
of the system as the dissipating signal comes back around on the
can. The resultant off-kilter delay cadence is largely responsible
for creating the sense of atmosphere without a strong “rhythmic”
element.

By adding feedback (REGEN) from the playback heads, and/or
combining the output of the two heads, the atmospheric nature of the
echoes increases.