General Asylum: It gets tricky here. by Trevor Wilson

Transistors have a VERY low output impedance. Much, much lower than the usually intended load impedance. For this reason, they can be damaged by too low an impedance. Unless they are fitted with external limiting devices, transistors will continue delivering more current, as the load impedance is reduced (assuming the power supply is capable). In real life amps, there are several things which make this not quite so, but you get the idea.

Tube amps are very different to this. Their internal impedance is considerably higher than transistors. For all intents, it is impossible for a tube amp to directly drive typical low impedance loads (leaving aside OTL amps, for the moment). They MUST have an output transformer to allow the correct reflected impedance, from the load, back into the tubes. If this is not done, then the power dissipation, within the tube, becomes too high. Triodes are MUCH more foregiving, than Pentodes, in this area. Triodes will tolerate a wider range of load impedances, since their internal impedance is much lower.

Trevor

Rage Audio