Hmm...I think Herman 'nailed it' when he wrote "Well, that should be enough information, mis-information (sic), and half truths to thoroughly confuse you."
Let's see if I can add anything useful with my just-enough-to-be-dangerous technical knowledge. (Just the other day I misstated on Audio Asylum the conversion of picoFarads to microFarads. There went my credibility! :-) )
First, generally, and compared with the impedances of typical loudspeakers, transistors are low-impedance devices and tubes are high-impedance devices. That's one reason the vast majority of tubed amps need output transformers and the vast majority of solidstate (SS) amps don't. Because the output impedance of a SS amp is MUCH lower than the impedance of a loudspeaker*, SS amps often are called constant-Voltage devices, since they can maintain Voltage as they flow more current as the load impedance decreases. Tubed amps cant do that. At rated output, tubed amps cant increase current flow (or at least current flow increases very little), so their Voltage output sags as load impedance decreases. Hence, their label as constant-current devices.
Cford is correct in his contrasting of MOSFET v. other transistors. As a MOSFET increases in temperature, its internal impedance increases, thereby resisting further increases in current flow. As a germanium-based transistor (GBT) heats, its internal resistance decreases, thereby allowing even more current flow, thereby heating more, thereby passing more current...ad infinitum. (That's called 'thermal runaway'.) I believe MOSFETS are inherently higher-impedance devices compared with GBTs and therefor are not able to flow as much maximum current as GBTs.
However, compared with tubed amps, MOSFETS are low-impedance devices, just not as low as GBTs, and therefor a MOSFET-based amp will still increase current as the load impedance decreases. Therefor theyre still constant-Voltage devices, just as GBT-based amps are.
* That characteristic is measured by 'damping factor', which is the ratio of the typical 8-Ohm loudspeaker to the output impedance of the amp.
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Let's see if I can add anything useful with my just-enough-to-be-dangerous technical knowledge. (Just the other day I misstated on Audio Asylum the conversion of picoFarads to microFarads. There went my credibility! :-) )
First, generally, and compared with the impedances of typical loudspeakers, transistors are low-impedance devices and tubes are high-impedance devices. That's one reason the vast majority of tubed amps need output transformers and the vast majority of solidstate (SS) amps don't. Because the output impedance of a SS amp is MUCH lower than the impedance of a loudspeaker*, SS amps often are called constant-Voltage devices, since they can maintain Voltage as they flow more current as the load impedance decreases. Tubed amps cant do that. At rated output, tubed amps cant increase current flow (or at least current flow increases very little), so their Voltage output sags as load impedance decreases. Hence, their label as constant-current devices.
Cford is correct in his contrasting of MOSFET v. other transistors. As a MOSFET increases in temperature, its internal impedance increases, thereby resisting further increases in current flow. As a germanium-based transistor (GBT) heats, its internal resistance decreases, thereby allowing even more current flow, thereby heating more, thereby passing more current...ad infinitum. (That's called 'thermal runaway'.) I believe MOSFETS are inherently higher-impedance devices compared with GBTs and therefor are not able to flow as much maximum current as GBTs.
However, compared with tubed amps, MOSFETS are low-impedance devices, just not as low as GBTs, and therefor a MOSFET-based amp will still increase current as the load impedance decreases. Therefor theyre still constant-Voltage devices, just as GBT-based amps are.
* That characteristic is measured by 'damping factor', which is the ratio of the typical 8-Ohm loudspeaker to the output impedance of the amp.
.