Amplifier circuitry-4 ohm vs 8 ohm

I believe tubes represent a more even profile of the harmonics, elevating the harmonics in the upper range better than solid state.

@emergingsoul 

This is incorrect. Tube amps tend to make more the lower ordered harmonics, the 2nd and 3rd, which is the source of their 'richness'. These two harmonics are at a high enough level in tube amps that they are able to mask the higher ordered harmonics, which are heard otherwise (if not masked) as harshness and brightness. The class D amp I'm playing at home does exactly the same thing, and since the distortion signature of any amplifier is the way that amplifier sounds its not a surprise that this amp sounds exactly like a very good tube amplifier.

I've looked at many tube amplifiers. Interested to know which solid state amplifiers achieve the same goal as tubes?

@emergingsoul In my experience, not very many! But I am playing a class D amp at home that has the same distortion spectra as you expect to see in a tube amp. It is at a lower level however. Most solid state amps look distinctly different when you look at their harmonic distortion.

Implied, but so far not stated specifically, is that the output tubes of an amplifier need to see a high load of several thousand ohms (the exact value depends on the tube and amp design.)  If you run an output tube straight into a speaker, whether 4 or 8 ohms, it looks like a dead short to the tube -- not good. 

And, straight from the tube, one is talking an output of up to hundreds of volts. Also not good for the speaker, which wants a lower voltage, but more amperage. 

We've been flying in the face of this for nearly 50 years making Output TransformerLess (OTL) amplifiers with a direct coupled output.

Somewhere along the way I realized that the distortion of the amplifier is also the 'sonic signature' that almost any amplifier has. So that meant if you could build a solid state amplifier with the same distortion spectra/signature, it would sound the same also. That proved out to be true.

So there are now solid state amps that allow you to leave the tube world behind without missing them for anything.

However,  I do recall that the late Roger Modjeski (RAM Labs Music Reference Audio) advocated “light loading” of amplifiers.

@charles1dad Roger's amps were designed so this technique could be used. Most amps are not. Roger's amps also used feedback and his recommendation relied on this. Your amps are zero feedback so it won't work with them. You will get less distortion from your power tube doing this, but the distortion generated by the output transformer on account of being improperly loaded will be far more than the gains you get from lightly loading the power tube.

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?

@dspringham Try it on both. If you have access to the impedance curve of the speaker, look at the impedance it presents to the amplifier in the bass region. That's where the energy is and will have the most effect on the interaction between the amp and speakers. If it seems more like 4 Ohms than 8, use the 4 Ohm tap...

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!