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Basic differences between valve and solid-state amps
Valve amps are high cost, high mass, electroniocally robust, but mechanically fragile.
Solid-state amps are low cost, low mass, mechanically robust, but electronically fragile.
Valve amps naturally tend to maintain constant power into a varying speaker impedance.
Solid-state amps can not maintain constant power into a varying speaker impedance.
Shorting the speaker lead has almost no dementral effect on valve amps.
Shorting the speaker lead can easilly destroy some solid-state amps.
Reactive impedance of cables, speakers and crossovers has minimal effect on valve amps.
Reactive impedance of cables, speakers and crossovers can disrupt many solid-state amps.
The valve amp output transformer acts as a buffer between amp electronics and the speaker.
Solid-state amp electronics are directly connected to the speaker.
Electronic faults within valve amps can not be transfered to the speaker.
Electronic faults within solid-amps (particularly DC latching) can destroy the speaker.
http://www.lenardaudio.com/education/14_valve_amps.html