Tube amps (except OTL types) use an output matching transformer. This transformer presents (relatively) fixed impedance (usually settable to 4, 8 or 16 ohms) to the speakers. The Matching Theorem (see any good EE circuits textbook) says that a source (i.e. amp) with fixed output impedance supplies maximum power when the load (i.e. speaker) has the same impedance.
Thus tube amps normally don't do well with demanding loads with impedances that vary across the audio spectrum. For example, when the speaker is attached to the 4 ohm tap on the amp, it gets maximum power when its impedance is also 4 ohms. Most speaker impedances vary across the audio spectrum, so power delivered to the speaker also varies with frequency. This effect is partly responsible for some of the tube sound.
Another way to look at it is that the inherent impedance of the transformer limits the current.
We can turn the matching theorum around, and ask what source (amp) output impedance delivers the most power to a given load (speaker). In this case, the lower the source impedance, the more power you get into a given load.
Transistor amps normaly don't have output transformers (there are a few exceptions). They tend to have very low output impedances. Many, (especially those built with massive power supplies and running class A) have output impedance much less than an ohm and can deliver large amounts of current into low impedance loads.
So if you need lots of current, solid state is the way to go. You can get there with tubes, but you need a lot of them, and massive (expensive) output transformers to go with them. They still supply maximum power only at the selected output impedance, though.
Thus tube amps normally don't do well with demanding loads with impedances that vary across the audio spectrum. For example, when the speaker is attached to the 4 ohm tap on the amp, it gets maximum power when its impedance is also 4 ohms. Most speaker impedances vary across the audio spectrum, so power delivered to the speaker also varies with frequency. This effect is partly responsible for some of the tube sound.
Another way to look at it is that the inherent impedance of the transformer limits the current.
We can turn the matching theorum around, and ask what source (amp) output impedance delivers the most power to a given load (speaker). In this case, the lower the source impedance, the more power you get into a given load.
Transistor amps normaly don't have output transformers (there are a few exceptions). They tend to have very low output impedances. Many, (especially those built with massive power supplies and running class A) have output impedance much less than an ohm and can deliver large amounts of current into low impedance loads.
So if you need lots of current, solid state is the way to go. You can get there with tubes, but you need a lot of them, and massive (expensive) output transformers to go with them. They still supply maximum power only at the selected output impedance, though.