OPT impedance vs. speaker

[kvk]

suppose you have a SE amp with a 5K:8Ohm OPT. Now lets say you run that with 16ohm or 4ohm speakers. What does that do to the OPT impedance? Does it make it 2.5K or 10K respectively?

[tubewade]

Impedance is reflected straight through an output transformer. If the primary impedance is normally 5,000 ohms with an 8 ohm load it will be 10,000 ohms with a 16 ohm load and 2,500 ohms with a 4 ohm load.

Wade

[kvk]

So what would then be the implications if one did run an 8ohm SE amp at 4Ohm or 16Ohm?

I would guess that there'd be less output at 16 Ohms?

At 4Ohms, I would guess there would be more output but also more distortion and more stress on the tube? If you were to just turn the volume a bit, would you still get acceptable performance?

[sorenj07]

2.5K SE is a reasonable impedance for some tubes, including 2A3's and 300B's.

[EC8010]

Loudspeakers are not resistors. Just because a driver claims 4 Ohm impedance does not mean it is 4 Ohms from 20Hz to 20kHz. Far from it. Here's a typical 150mm "8 Ohm" driver. I use the 4 Ohm tap for everything.