If you need to make a mismatch it's better to have the speaker impedance lower than the amp's impedance.

Why is that?

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f the amplifier was designed for a 10k load (4 ohm on 4 ohm tap) and you put a 4 ohm load on the 16 ohm tap, your load becomes 2k5. If you put a 16 ohm load on the 4 ohm tap, your load becomes 40k.

That's my understanding.

Let's take an OPT with 5k ohm primary and 4 ohm secondary.

That's a turns ratio (primary:secondary) of about 35.4:1

That turns ratio never changes. There are always 35.4 turns on the primary for every one turn on the secondary.

If we double the load on the secondary to 8 ohms, the reflected impedance of the secondary back to the primary stays in the same ratio. So doubling the secondary impedance will double the primary impedance. That 5k:4 transformer will now be working as 10k:8.

When you connect a triode to the primary, the max power will be transferred to the load when the primary impedance is 2X the triode's rp. Let's say that triode has internal plate resistance (rp) of 2.5k ohms. In that case, a 5k ohm primary impedance will get you the max power out from that triode. If the load is 4 ohms, then you choose an OPT of 5k:4 ohms impedance (35.4:1 turns ratio).

When you increase the load to 8 ohms and connect that to OPT's 4 ohm secondary tap, the primary impedance increases to 10k ohms. Now the Zpri is 4X the triode's rp. That will operate the tube with more damping (a higher impedance load), yielding lower distortion, but also lower power output into that 8 ohm load.

If the output tube being loaded by the OPT primary is a pentode, then things are more complicated. But you can see from the above that there is a *range* of impedance loading choices with any output tube and OPT. It's not like there's one 'magic' primary impedance that will yield perfect results, and deviating from that will cause catastrophic failure.

This graphic shows how power output and distortion vary with primary impedance in a Push-Pull class AB1 pentode amplifier.

You can see that there's a sweet spot where you get the most power output with a 6k primary impedance. But the lowest distortion happens if a 7.6k primary impedance is chosen, but as you can see, you won't get as much power output. You get to choose your compromise between max power out vs. lowest distortion.

I hope I explained that reasonably well. Trying to explain these things is a really good test of one's understanding...

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