Tube Amp. Power ratings

In general you will be noticing more detail with a more relaxed presentation with tube gear. As you noticed, power is harder to make with tubes so they are often less powerful for the money. You have to use a speaker that is designed for tubes- tube amps tend to make constant power with respect to load rather than constant voltage of most high quality transistor amps. This appears to be a matching issue, but it is more than that, take a look at

http://www.atma-sphere.com/papers/paradigm_paper2.html

If you use a speaker intended for tubes with a transistor amplifier, you will get tonal aberrations just like you will if you use a tube amp to drive a speaker intended for transistors. But overall, if you have a setup where the speaker is optimized for the tube amplifier, the presentation will be more lifelike and musical, as tubes in general obey more of the rules of human hearing than transistors do. Its something that you hear right away- its not subtle.

Porziob, the fact that there are OTLs that can handle four ohms is not a matter of debate. Some 20 years ago, your assertion was common knowledge, but the succeeding decades have produced amplifiers that fly in the face of that. Also, that tubes are more revealing is a demonstration issue, but one so apparent that it is easily settled in 5 seconds flat- its not subtle. If you've not been exposed to such technology yet, perhaps its time?

I should point out that if you are considering a tube amplifier, it really is a matter of the right tool for the job- you wouldn't use a hex wrench to tighten a pipe, you use a pipe wrench for that. In the case of tubes- *any* tube amp, the investment dollar in the amplifier will be best served by speakers that are at least 8 ohms rather than 4, all other things being equal, regardless of the technology of the tube amplifier.

IOW the point here is not the ability to arc-weld, as that has nothing to do with hearing- its all about delicacy and authority manifesting in the same system at the same time. This is the pervue of tubes, which is lost on 4 ohm speakers ( which really did not appear until transistors came in). 16 ohm speakers are starting to appear again, now that there are more tube equipment manufacturers in the US now than there was in 1956...

Cincy_bob, 'chip amps' have been around in one form or another since the late 70's. The issue with them is that the form factor limits the amount of heat they can dissipate and the amount of current that they can pass. So you can damage them if you try to operate them on 4 ohm speakers. They require feedback for operation, a fundamental difference between them and tubes, which can be designed to operate without feedback.

Negative feedback enhances (slightly) high-order odd harmonic distortion, a form of distortion that the human ear can detect in levels of 100ths of a percent. This we hear as harshness or at best an unnatural sheen on top.

Jamnesta, yes. The degradation in performance of any output transformer on its 4 ohm tap is audible and measurable. You can often loose an octave off of the bass end, and often too the actual power output drops as the transformer is less efficient.

Cincy_bob, the way out of the conundrum is to choose a speaker with a flat impedance curve. Such a speaker, if 8 ohms, will be about equally friendly to either technology.

Ckoffend and Jaffeassc, I would try it both ways but I would expect that the amp and its power tubes would be more comfortable on the 4 ohm tap. The transformer actually does what its named for- it transforms impedance.

Here's how that works:

Let's say you have a set of power tubes and they are expecting 3000 ohms plate-to-plate. Now if you load the 8 ohm tap with a 4 ohm load, you have just cut the impedance in half, so the power tubes are now seeing 1500 ohms plate to plate. The result will be higher distortion and likely significantly less output.

There are output transformers out there that are not that efficient on their 4 ohm taps, so with some amps the 8 ohm tap may actually be preferred, but most other tube amps will likely be set right on the 4 ohm tap. This is of course assuming that the match that the power tubes want to see is in fact satisfied by the windings of the transformer, which is often not the case as the combination is an approximation. So I would expect that there are also some output transformer situations where the 4 ohm tap is getting the tubes closer to the ideal.

If the 4 ohm tap is the correct tap, then loading the 8 ohm tap with 4 ohms will cause the power tubes to work harder, as more of the power they produce will be dissipated inside the tubes rather than the load. So there is a certain gamble in trying 4 ohms on the 8 ohm tap, although tubes are forgiving enough that its not a risk to try- more of a risk to keep it set up that way.

This can be a big topic so I am glossing over a few things, but that's it in a nutshell...