why do SET tube amps output wattage decrease with lower impedance loads
(speakers) in comparison to SS amps wattage output staying the same or
even increasing with lower impedance loads?
@sdguyer87 The short answer is that an SET has a high output impedance that is not that dissimilar to the speaker it drives, while most solid state amps have an output impedance that is a very small fraction of the speaker load.
The latter allows the amp to behave as a 'voltage source', meaning that it can make the same voltage regardless of the load it drives. So if it is making 1 watt into 8 ohms its 2.83 volts; it will make the same voltage into 4 ohms, thus it is making 2 watts.
An SET typically operates without loop feedback and the tendency is thus to behave more like a 'power source', although it will only be an approximation. This does mean though that it will try to make the same power into 8 ohms as it does into 4. In reality, because the output transformer transforms impedance, into 4 ohms the power tube will have a load that is half of what it should have, and so will make far less power, unless the 4 ohm tap is used which restores the proper load for the output tube. However, output transformers are typically less efficient on their lower impedance taps, so its very likely that it will make slightly less power into 4 ohms with the 4 ohm tap, and likely also will be more restricted in the bass region- a loss of an octave of response is commonplace. The distortion will likely be higher too.
