[Phil Allison[_4_]]

Mike Rivers wrote:



But tube amplifiers are different from solid state
amplifiers in many ways. A solid state amplifier has a very low output
impedance, which makes the voltage that the speaker sees pretty much the
same for an 8 ohm or a 4 ohm speaker. But for the same applied voltage,
the 4 ohm speaker will draw twice as much current, and that's what makes
the amplifier sweat.


** Typical SS amps for home hi-fi have output impedances around 0.1 ohms.

The speaker lead adds maybe 0.3 ohms to that - so 0.4 ohms all up.


A tube amplifier has an output transformer which has a higher source
impedance, and they make an attempt to match the source (transformer
output) impedance to the load impedance by putting a couple of taps on
the transformer secondary.


** The output impedance of a good quality tube amp is around 0.5 to 1 ohm when used at the 8 ohms setting.

For a given input signal level, the 4 ohm tap provides a lower voltage
than the 8 ohm tap.

** But with 40% more current available.


You get maximum power
transferred from the amplifier to the speaker when the load and source
impedances are equal.


** More precisely, when the two are *matched* - so an 8ohms load on the 8ohm setting and so on.

However, actual speaker impedance varies widely over the audio range so it is only gonna be "matched" in the mid band - around 200 to 500 Hz.



..... Phil