Tube power vs Solid state power, how much is enoug

Tube amps tend to clip 'softer' than SS amps. That is why you can play a tube amp as loud as a larger SS amp. Look at the distortion specs of both amps at full power and also the spectrum of that distortion. Generally the SS amps will come on strong with higher order odd harmonic components as they exceed full power. Tube amps generally have more harmonic distortion; but it builds more slowly and tends to be dominated by 2nd order even components. These are present in musical instraments and generally don't sound harsh or displeasing to the ear. Also, since they are already there in the music, it takes more for the ear to notice the increase. Whereas the higher order odd components tend to stand out and sound bad.

A watt ain't a watt ain't a watt. This is true even though it is supposed to be and there are set electrical parameters as to what constitutes a "watt".

Tubes and SS devices work on different principles. To put it in plain English, almost all SS designs are voltage limited whereas Tube amps are current limited. Some designs will work better with one type of amplification whereas that same amplifier may be completely lacking with a different type of speaker load.

I can explain why this is so if you folks are interested. Sean
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I have very high efficiency speakers and there is more to it than that. Tubes generally fair better with a higher resistance load and are less prone to distort (impedance matching). Secondly the dynamics of the music change the load as you play and how loud you want to play changes the tendency for audible distortion. I have found that certain drivers within a speaker respond differently despite a nominal load as well. If your speakers have power-hungry woofers beware. Finally you should be very clear the power as stated in RMS wpc tells you only a small part of what an amps overall "power" is. The current capacity is very important. Since I am not an electrical engineer I can't tell you all you need to know, your ultimate test is to listen. Therefore Blindjim your choice of speaker and amp in combination is critically important. I use a 40wpc tube amp on lower 88db/W/M efficiency speakers (beauty) and a true 100 wpc solid state on very high efficiency speakers (beast) and get the effect I want from those two systems.

Sean, please do.

Sean is right and Sean is wrong. A watt is indeed a watt. Also, many years ago the Federal Trade Commission regulated the way manufacturers rated the power of their consumer audio equipment, so generally, one at least gets the "100 watts into 8 ohms from 20 Hz to 20KHz at no more than 0.1% harmonic distortion" kind of rating. (Previously manufacturers rated amplifier power any way they wanted and usually they didn't tell you enough.)

However, Sean is correct about tubed and solidstate amplifiers behaving much differently driving different loads. Because tubed amps have output transformers, they have relatively high output impedance, while SS amps generally have very low output impedance. Within its construction limits, a SS amplifier will continue to increase its current output and maintain its voltage output (hence increasing its power) as the load impedance decreases. This means that about-any SS amp that's driving 1 watt into 8 ohms will drive 2 watts in 4 ohms, 4 watts into 2, 8 watts into 1, 16 watts into 1/2 ohm, etc. Of course, the higher amount of power (and current) we start this exercise with, the quicker the amp will reach its limitation on current flow. 'High current' amps will indeed increase power as the load impedance decreases, and the good ones will double rated power at least once. For instance, an inexpensive amp may have only one power rating, usually into 8 ohms. If it doesn't have another rating of higher power into 4 ohms, it’s not a high-current amp IMO. Some will have more power at 4 ohms but not double the 8-ohm rating. The better amp will indeed double its power into 4 ohms, and the really high-current designs will double again into 2 ohms. For example, my humble little (but HOT-running) Lazarus hybrid amp (with SS output stage), is rated at 50 watts per channel into 8, 100pc into 4, and 200pc into 2 ohms. It costs lots more money to design and build an amplifier that will do this.

Tubed amps can't do that. A tubed amp will increase power slightly as load impedance decreases but not by much.

But still, a tubed amp will sound somewhat better when driven to deliver power beyond its rating (ie, ‘at clipping’) than a SS amp will; many people say that a tubed amp clips more ‘gracefully’. That’s why many of us say that generally one can get away with a tubed amp of half the rated power compared with a SS amp. However, the lower the impedance of the speaker (and hence the more current the amp has to dump into it to play music as loudly as the listener desires), the less likely would a tubed amp be able to cope gracefully and sound good. Of course, each amp of either type will behave differently and sound different under various real speaker loads.

Also, Gs5556 is absolutely correct, and lots of us pay little attention to the relative efficiencies (sensitivities) of speakers. Every 3dB of higher or lower sensitivity requires double or half the power to drive to equal loudness. My 96dB-rated Kindel PLS-As require ONE-QUARTER the amplifier power than a 90dB-rated speaker to drive to equal loudness. That’s why my ‘little’ 50wpc Lazarus amp will drive them to deafening levels. I don’t wish to belabor this point, but a not-untypical 87dB-rated speaker requires FOUR-HUNDRED watts to generate the same loudness as my 96dB/50-watt combination.

So Blindjim, start your evaluation process by picking the speakers first, understanding their sensitivity ratings, and then picking some amplifiers to listen to. Unless the speakers’ impedance dips below, say, 4 ohms over a significant part of the frequency range, a tubed amp will probably do very well.

And my answer your original question is 'enough to avoid amplifier clipping about 99% of the time'. Yes, that's not very useful, but YOU have to answer it, not us. :-)