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"Tube/transistor "HYBRID" amplifiers

Tubes are often used to impart characteristics that many people find
audibly pleasant to solid state amplifiers, such as Musical Fidelity's
use of Nuvistors, tiny triode tubes, to control large bi-polar
transistors in their NuVista 300 power amp. In America, Moscode and
Studio Electric use this method, but use MOSFET transistors for power,
rather than bi-polar. Pathos, an Italian company, has developed an
entire line of hybrid amplifiers.

To demonstrate one aspect of this effect, one may use a light bulb in
the feedback loop of an infinite gain multiple feedback (IGMF)
circuit. The slow response of the light bulb's resistance (which
varies according to temperature) can thus be used to moderate the
sound and attain a tube-like "soft limiting" of the output, though
other aspects of "the tube sound" would not be duplicated in this
exercise."

http://en.wikipedia.org/wiki/Tube_sound