Many tube amplifers have several output taps for various speaker impedances. Typically 4 and 8 Ohms sometimes also 16 Ohms. There is a lot of confusion about which is the right tap to use. With this article I try to shed a bit of light onto this subject.
There is no technical standard which defines the parameters that would qualify an output as 4 or 8 Ohm. The difference between the two is the output impedance and matching to the output stage. Different amplifier topologies and philosophies will yield very different output impedances though. On the 8 Ohm output for example the output impedance can be anywhere between 1 and 4 Ohm for sensible designs. Even outside these limits. For the 4 Ohm tap it will be between 0.5 and 2 Ohms. As you can see there is some overlap in these ranges. So what one amplifier designer would label as a 4 Ohm output, another one would find suitable for 8 Ohm speakers only. So this can be understood more as a recommendation, not a hard rule which terminal is to be used with which speaker.
Then look at the impedance of a typical loudspeaker. The nominal speaker impedance is only a rough approximation. The impedance typically varies widely over the frequency band. A 8 Ohm speaker for example might have an impedance close to 8 Ohm somewhere in the midband. In the bass region you will typically see resonance peaks at which the impedance can rise to 20-30 Ohms or even more. In the midband and treble there can be dips significantly below 8 Ohms. Dips to 5-6 Ohms are not uncommon. Only few speaker designers linearize the impedance. Again there are no standards which define how the nominal speaker impedance is derived from the impedance curve. So again this number is widely subject to interpretation and different speaker designers will declare different nominal impedances.
Due to these two uncertainties in the impedance numbers, there is no need to slavishly connect 8 Ohm speakers to the 8 Ohm tap only. Experimenting makes sense. Connect your speakers to the tap which sounds best to you. Most often hooking a 8 Ohm speaker to the 4 Ohm tap can yield some improvement. Due to the lower output impedance of this tap, the impedance variation of the speaker will have less impact on the frequency response as with the higher output impedance, the latter can actually cause some coloration. The lower output impedance will also mean more 'control' of the amp over the speaker (better damping factor). However some speakers actually sound better with a smaller damping factor. The disadvantage of such mismatching will be that the maximum possible power output of the amp gets a bit reduced. But this is negligible in most cases.
If you have a speaker with a bi-wiring terminal, or better yet, if you build your own speakers and have full control over the crossover, there are more ways to experiment with output taps. For transformers, the relationship between winding and impedance ratio follows a square law. Besides impedances the transformer also transforms voltage and current. The ratios of voltage and current correspond linearily to the winding ratio. What does this mean? At a tapped secondary, the voltage at the midpoint (center tap) will be half the voltage which is seen across the entire secondary. The reflected impedance however will be only one fourth.
This means that in case you have 4, 8 and 16 Ohm output taps, the 4 Ohm tap is actually the center tap of the secondary winding of the output transformer. There is the same output impedance, output voltage and current across 0-4 and 4-16.
Now if you have a speaker with separate terminals for low frequency and high frequency, you can utilize the full winding, even if the speaker is not rated 16 Ohms. For this to work properly, low and high frequency sections need to be completely isolated. This can be checked with a Ohm meter. There should be an open between both ground terminals. Now you can connect the low frequency section between 0 and 4 Ohm taps and the high frequency section between 4 and 16 Ohm. Polarity needs to be observed. See the illustration below, how this looks like.
No more unused parts of the output winding, dangling in the air! You can even use the taps to attenuate the high frequency part if necessary. The output voltage at the 8 Ohm tap is about 3dB above the output voltage on the 4 Ohm tap. This is neglecting load and output impedance, depending on the actual output impedance and speaker impedance the difference will be more like 2dB or less. Calculation or measurement is necessary to get the exact difference. In most speakers the woofer has a lower efficiency compared to the rest. So tweeter and/or midrange need to be attenuated which is usually done by a resistor divider network. With a multi tapped output transformer this adaption of the levels can probably be done without resistors, if in a given system a certain combination gives the right attenuation.
Below some possibilities how LF and HF part can be hooked up to achieve different levels of attenuation:
The two alternatives in the first row provide 6dB and 3dB raw voltage difference (HF section attenuated). With real life impedances probably more like 4 and 2 dB. In the two examples in the bottom part, HF is attenuated by about 4.5 and 7,5dB (unloaded). So there are quite a few possibilities to adjust the level of the HF part through this technique.
This leaves quite a lot of room for experimentation and exploitation of all the transformer taps which might otherwise be unused. The sonic result of this technique heavily depends on the amplifier, transformer and speaker used. Keep in mind that this is playing with mismatching. In some configurations it might not work well. Also the full output power of the amplifier will not be achieved with this. If the system has enough headroom it is worth playing with this though.
In addition to purposely mismatching parts of the speaker to the amp to achieve the required attenuation, this technique can also be used to achieve correct matching, when woofer and tweeter have different impedances. For example if a 8 Ohm woofer and 4 Ohm tweeter are combined in a speaker, both can be hooked up to appropriate parts of the secondary winding, through their respective crossovers. If 4 Ohm woofers are used with 8 Ohm tweeters, the tweeter impedance can be adapted to the 4 Ohm with a parallel resistor. Since tweeters are typically more efficient, this does not hurt. The other way around it is more difficult, you would not want to bring a 8 Ohm woofer impedance down with parallel networks to match a 4 ohm tweeter. Creative usage of output taps could make this more easy.