The transformer does indeed transform impedance. So if the power tubes want to see 3000 ohms plate to plate, then a specific tap on the secondary side is going to be the one that works. So if you load the 4 ohm tap with an 8 ohm load, the load on the tubes will be significantly higher so they won't be able to make full power and the transformer will be able to ring (distort). Plus, the inter-winding capacitive effects will increase, degrading the frequency response.
If you put a 4 ohm load on the 8 ohm tap, the tubes are now going to see something significantly lower, and they will not be able to make full power, plus there will be more distortion.
For this reason the taps have to be used correctly to prevent distortion and maintain bandwidth.
Not all tube amps are designed to be voltage sources. In fact, there is a strong argument for current sources, not the least of which is that they are likely to sound better. If you measure such an amplifier using voltage source rules, the amp may well measure poorly, although it may sound fine.
for more info see:
http://www.atma-sphere.com/papers/paradigm_paper2.html
If you put a 4 ohm load on the 8 ohm tap, the tubes are now going to see something significantly lower, and they will not be able to make full power, plus there will be more distortion.
For this reason the taps have to be used correctly to prevent distortion and maintain bandwidth.
Not all tube amps are designed to be voltage sources. In fact, there is a strong argument for current sources, not the least of which is that they are likely to sound better. If you measure such an amplifier using voltage source rules, the amp may well measure poorly, although it may sound fine.
for more info see:
http://www.atma-sphere.com/papers/paradigm_paper2.html