Amplifier circuitry-4 ohm vs 8 ohm

The only place I know where this happens is in tube amps with transformer outputs.

The idea is to keep the power output the same, so the 4 Ohm tap has less voltage, more current, same power as the 8 Ohm.

Each tap is a secondary winding on the output transformer. The primary voltage is transferred to each of the 8-ohm and 4-ohm secondary windings equally. The impedance is set by the turns ratio of wires between the primary and the 4 ohm tap, and the primary and the 8 ohm tap. The relationship is the square root of the impedance so to get an 4 ohm impedance, you wind 1.44 times as much wire to the 4-ohm tap (square root of 2) as you do for the 8-ohm tap. This will set both the voltages the impedances the speakers the speaker sees when connected. 

      YEP (GS beat me to it)!

       Here are two references, that may illustrate/shed more light on the subject:

                                   and (under ’Output Tranny Types’, parameters):

                      https://education.lenardaudio.com/en/14_valve_amps_5.html

The single amplifier at some point splits the signal going into either one of these Ports. What differences are there in the circuitry?

Maybe this will help me better understand the difference between these two taps. I believe 4 ohms is a wider more open path for voltage to flow. So when you’re speaker attempts to go lower, which requires more power, the 4 ohm more easily allows this to happen with a better outcome. Or maybe I got this wrong.

@emergingsoul 

You did.

As pointed out, the one output transformer for each channel has taps in a single winding for 4 and 8 Ohms. If the 4 Ohm tap is used with an 8 Ohm loudspeaker, the power tubes will be loaded at too high an impedance (transformers get their name from the fact that they transform impedance, and that goes both ways) and so will not make nearly as much power as they are supposed to.

If the 4 Ohm tap is loaded with 8 Ohms the transformer will also ’ring’ which is to say it will make distortion of its own. If the amp employs feedback, it might be able to compensate for this. But it will be lower distortion if the transformer is simply loaded correctly- so if an 8 Ohm speaker put it on the 8 Ohm tap!

If a 4 Ohm load is put on the 8 Ohm tap, the transformer will again be improperly loaded and so will the power tubes- they will have a load too low, causing some of the power they make to be dissipated in the tubes themselves, causing them to run hotter! In addition, the output transformer will be rolled off in the highs, although feedback might be able to compensate for that. However the power tubes will make more distortion, so you can see this is a Bad Idea. If a 4 Ohm load, use the 4 Ohm tap!

@atmasphere 

What if the speaker has a nominal impedance of 6 ohms? Which tap (4 or 8) should be used since the load is right in the middle of the two windings?