That "tube sound" and power ratings

"Probably... A lot of what people like is the compression and fatness that occurs when an underpowered tube amp is over driven"

That's a very good point and a great example of how the best technical performance in terms of accuracy does not always win.

As I understand the question, you are seeking to determine if in general it makes sense to prefer a lower powered tube amp to a higher powered tube amp (assuming that both amps can provide sufficient watts for the application), due to an expectation that the lower powered amp may, under typical conditions, tend to distort more than the higher powered amp. And thereby provide sonic characteristics that are more "tubey" and less solid state-like than would otherwise be the case.

That is a logical question, but I think it focuses MUCH too narrowly on one single factor in tube amp sonics for any answers to be useful. There are simply way too many variables involved in the design and the performance characteristics of an amplifier, and in its interaction with the technical characteristics of whatever speaker is being used, for a useful answer to be defined. Certainly among different makes of amplifiers. And even when considering amps from a given manufacturer having different power ratings, that are otherwise generally similar in design, I see no reason to expect much consistency among the product lines of different manufacturers as to whether the higher powered or lower powered amp would be preferable. Or even much consistency across a variety of speakers when a comparison is performed between a given higher powered and lower powered amp, as a given tube amp can sound quite different depending on the magnitudes and phase angles of the impedances of the speakers it may be used with, and especially on how those parameters vary as a function of frequency.

Also, the question focuses on just one kind of distortion, harmonic distortion. There are other kinds, e.g., intermodulation distortion, transient intermodulation distortion, crossover distortion, etc. Some of these are neither generally specified nor even measurable in a standardized manner. And all involve a complex signal-dependent mix of different effects and different degrees of effects at different frequencies.

What I would suggest as the most useful line of inquiry, when you reach that point, is that you start a thread indicating the particular speakers that would be used, your budget range for amplification, the room size and listening distance, some idea of the volume levels you prefer, and (especially) whether or not your listening includes recordings having particularly wide dynamic range (i.e., particularly large DIFFERENCES between the volumes of the loudest and the softest notes, such as is often the case with well engineered, minimally compressed, classical symphonic music). The latter factor will dramatically affect how much power you need. Providing that information will get you many knowledgeable and useful suggestions from the members here, from which you can develop a “short list” of candidates.

Good luck. Regards,
-- Al

Zkzpb8 & Mapman - thanks!

Almarg - very helpful, thanks! Yes, my post is focused narrowly on this one question, but I'm not. When I listen I listen for what pleases my ear. If this is one of the things that does or does not then I'll note that fact and keep listening. I'm not really looking for specific amplifier suggestions, at least it didn't occur to me to do that. If I do it will be in another post, but I doubt that'll be necessary.

Hazyj, There is something to the idea that smaller amps sound better. In the case of push-pull tube amps, this has almost entirely to do with the output transformer itself. The reason for this is that the more power the output transformer can handle, the less bandwidth it has. So in the larger designs, usually the designer has made a tradeoff based entirely on what he thinks is important.

If the amp employs negative feedback, usually the high frequency bandwidth is the area that is sacrificed. This is because there is not a lot of energy at high frequencies so its possible to get away with this if you also run negative feedback to help output a little with the voltage response of the amp which will be degraded (another way of looking at this is the output impedance will be higher at the area of the rolloff and beyond).

In the old days, 60 watts was about the limit where the output transformer could make the bandwidth required to really be hifi. That bandwidth BTW is well past 20Hz-20KHz; its nice to get response up to 100KHz if you can so phase shift at 10KHz is not adversely affected. This really helps with the presentation of the soundstage, depth in particular.

These days designers seem to have pushed that power level up to about 100 watts before bandwidth really starts to suffer.

Now the problem with push-pull is often that there is a phase-splitter circuit that introduces some distortion. This is not true of all P-P amps, but it is true of most of them. This generally is not an issue until you get into the lower power regions of the amplifier, at which point the distortion of the phase splitter comes into play. For this reason a highly efficient speaker (+98 db or so) may not be the best choice with a plus-100 watt amplifier as you may never get the amp out of that region of higher distortion at lower power levels.

SET amplifiers get around this problem by not having a phase splitter (that's how we do it too although our amps are push-pull). This allows them to have a distortion characteristic that becomes unmeasurable as the power decreases to zero. To really take advantage of this, you really do need a high efficiency loudspeaker! The reason is two-fold with SETs- first, that bandwidth limit discussed with the OPT of P-P amps is more profound with SETs, the practical upper limit being only about 7 watts before bandwidth is degraded (for this reason many SET designers don't bother trying to get the bass right in the OPT as the speakers that have that kind of efficiency don't play the bottom octave either). The second reason is that you never want to push an SET past about 20% of full power or else the higher ordered distortion products come into play (when this happens, its usually on transients, and because the human ear uses the higher orders as loudness cues, the result is that the SET sounds a lot more dynamic than it has any right to because the loudness cues are occurring on the transients. You read about this 'dynamic' character with SETs all the time, but its an indication that the speaker used is not efficient enough).

Where is is all going is that if you want to really get the most out of your tube amplifier investment dollar, the selection of the loudspeaker is really important!

Something else to be aware of: Tube amps make more power and run more efficiently into higher impedances. You really don't want to make **any** amplifier (tube or transistor) work all that hard- the harder you make it work, the more distortion it will have. So you want to avoid 4 ohm speakers generally speaking- the amp will make less distortion into 8 or 16 ohms. This has nothing to do with whether the amp employs negative feedback or not- and it has everything to do with the fact that OPTs operate more effectively (wider bandwidth, lower distortion, less amplifier power converted to heat) into higher impedances. This is why you are seeing more 16 ohm speakers on the market these days- designers are slowly figuring this out.

Atmasphere, who out there today is making good quality 16 ohm speakers? With no impedance dips much below that I mean.

A google search on 16 ohm speakers turns up little at least on teh fist page.

WHat are some good ones at various price points? Which ones are full range?