You might like to look at the August Stereophile, which has an article on this.
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In regards to the ability to deliver current into a low impedence, that is THE weakpoint of a tube amp. Classic(not usually in the real world) solid state amps doubles power as the impedence is halved(that 200W could act like a 400W amp). A tube amp puts out the same power(unless it is an OTL, in which case it would put half as much power into half the impedence). Power ratings(the ones that we talk about) are usually specified at 8 Ohms. And with a speaker that is not a difficult load, the analogy you stated is usually accepted. But with a difficult speaker, the tube amp will never do it justice. Now, if you use one of those monsters from VTL, etc. with 16 or 32 output tubes, you can drive even a difficult speaker. But they are REALLY expensive, and get even more so when you retube(every 3500 - 5000 hours). Assuming you could buy a matched pair of 6550s(discounted, via the internet) for $60, times 16 = $960! But, if I was using a tube amp, I wouldn't not give it a difficult load. There are plenty of easy to drive speakers around today.
I have heard that a low impedance speaker, like an electrostatic, it will eat tubes like there's no tomorrow. On the topic of equivalent power. I've had 100w tube, 160w tube, and a 185 w solid state amp in my system. I can attest that the solid state is MUCH more powerful in terms of controlling the speaker. They are not even close. I do not drive my speakers to high levels, so I cannot comment on clipping points. I also note that when people talk about a tube amp having good bass, it is in terms of a tube amp. I had two high quality tube amps, and neither had even a shadow of a solid state amps bass control. The amps were AR vt100m2, Sonic Frontiers SFM-160's. For solid state I have , a McCormack DNA1/AGold. The SFM-160's actually had internal jumpers to that you could set for 8,4,2 ohm loads. The vt100 had 4 and 8 ohm terminals. I have also had my old 125w Denon receiver hooked up to the speakers, and although the sound quality was lower, the bass was still more powerful than either of the tube amps. I've driven Vandersteen 2's and 3s (8 ohm), and virgos (4 ohm) with these setups. The vt100 seemed to be a little wimpy with the vandersteen 3.
I understand the theoretical differences which suggest solid state should be better than tubes in the bass, and ditto for low impedence speakers. But, in one of my systems, the best amp for bass, and for the Martin Logans and Thiels they drive is a tube amp. This is despite trotting many solid state amps through the joint (Krell, Classe, Aragon, Ocean, Lewitt, Plinius etc). I suspect the bass is better with tubes because the floor is suspended across spans of about ten feet - meaning it sucks bass somewhat. Using high-damping factor solid state amps the bass sounds almost swallowed. The tube amps (admittedly very good ones) have more realistic bass, with tightness, extension and air. I do not make this point to negate what has been said, only to point out that some situations prove to be the exception to the rule.
Thanks for the input, guys. I've recently purchased a pair of Martin Logan CLS II's (non Z version). Not set up yet due to a recent move. My McCormack DNA 1 Deluxe has done a good job of driving my Apogee hybrids and I'll try it out with the CLS's. Since the CLS's don't go to high Db levels or very low in the bass, my thought was that maybe something like a VT100 might do a better job in the mids and provide more air -- the liquidity of a good tube amp might be a good match for the transparency of these speakers. I guess the only way to find out is to try both. Audio Research is coming out with a MK III version of the VT 100, so there will probably be good deals available soon on MKII's. Yeah, replacing the tubes is a hassle and an expense, but if the sound is right I can live with that (I think). But I still question this amp's ability to deliver the required current to these very difficult to drive speakers (impedance can dip to .5 ohms). Thanks again for all of the information. Jim
There are speakers that present a difficult load at both ends of the (frequency) and impedance range. Soundlab Ultimate Ones are are an example I am familiar with. The are about a 30 ohm load in the deep bass, but only 2.5 ohms in the extreme highs. The power provided by most solid state amps is "halved" at each doubling of the load above 8 ohms. A 400 watt transistor amp at 8 ohms, would (in theory) produce 200 watts at 16 ohms and 100 watts at 32 ohms. This 100 watts of power this transistor amp is reduced to is made worse, as it is in the "power hungry" region of deep bass. In addition to that, the electrostatic panels are capacitive in nature, and have a fair amount of reverse electrical feedback that the amp must tolerate. A good tube amp, OTL or transformer coupled will often out drive even a huge transistor unit in this situation. I am able to play my Ultimates to almost live level in a large listening space, with a 25 watt SET tube unit. My point is the same as Redkiwi was making, there are situations that prove the exception to the rule.