Tube Watts vs. Solid State Watts - Any credence?

Geof will then rewrite all the textbooks on the subject as well I'm sure. :^}

...but can you give algebraic or geometrical representation so everyone can understand?

Czarivey, good question. Yes, they're better, too.

Watt's on second.

Geoffkait,
How about Amperes, Volts and Ohms are they also better?
Ampere, Volt, Ohm and Watt were dudes that named these units of measure. I would suggest you to provide quantitative answer to your hypothesis by deriving your own unit of measure by how much of your units TubeWatts are better than SSWatts.

I should have written that sentence differently pointing out that output power depends on the topology of the tube - triode vs. ultra-linear vs. pentode with the output power going progressively higher as one moves from triode to pentode.

Really it depends on how many tubes are used. Our MA-3 makes over 500 watts and its entirely triode, yet the H/K Citation 2 is pentode and only makes 60 watts. I think what you were trying to say in this passage is that pentodes **usually** pack a lot more power per tube than triodes usually do (there are triodes out there that make a lot of power; more than most pentodes).

Ultra-linear is a means of approaching triode linearity while retaining the power output of a tetrode or pentode (either can be used in ultralinear). The use of tetrodes, pentodes, triodes and ultralinear operation otherwise has nothing to do with power at all.

i think he's saying the same thing Atma-sphere & I & others have already written which is: a watt is a watt but each amp reacts differently with each speaker.

Actually I was more commenting on strictly how the amps distort- tubes interact favorably with human physiology while generally transistors do not (lower ordered harmonics as opposed to the dreaded 7th harmonic). This interaction can be quite profound and hard to turn down when you hear it!

A few years ago my band was at a show in Chicago. There were a lot of bands on the bill and to save time with each one shifting gear on stage, we offered to 'backline' our bass amplifier- a 400-watt Peavey ('backlining' is the process of having some equipment left on stage that all the bands would use during the show). One of the bands had a 500-watt Orange bass amplifier, which is class D. He wanted the extra power. He was quite blown away when I pointed out that a class D 500 watt amplifier would in no way keep up with a 400 watt tube amp on stage. He tried it and agreed- but had to go back to the class D amp as the Peavey made far more bass energy than he was used to- he was used to the thinner sound of the class D amp and was a little panicked that he would not be able to get the right sound (they were going on first).

Watts are not Watts. The reason tube amps have better Watts than solid state amps is pretty simple. It's because they generally have more dynamic headroom than solid state amps. Thus all things being equal tube amps won't clip as soon as solid state amps. Case solved.

10-12-15: Beavis
Are you trying to tell tube/solid state amps measure RMS differently?

Since Mapman has not answered till now & if I may be permitted to reply in his stead understanding what he is trying to say then.........
no, he's not saying that s.s. & tube amps measure RMS differently. That simply cannot happen - RMS is RMS no matter which amp.
i think he's saying the same thing Atma-sphere & I & others have already written which is: a watt is a watt but each amp reacts differently with each speaker. And, it is this interaction between amp-speaker that determines when & how much an amp will distort. This, in turn, will give the illusion in some cases that tube watts are more powerful than s.s. watts (they really are not).

10-12-15: Trelja
Another thank you, Bombaywalla!

welcome Trelja.

Another thank you, Bombaywalla!

For the most part, I've never much cottoned to ARC (or, CJ) products. As time moves forward, I tend to like the newer ARC products even less. Still, outside of the commentary OTL and a certain slant on the power supply, I came away more than impressed with how Mr. Johnson thinks and feels. Call it new found found respect.

Anyway, the contrarian part of my post is that a watt is NOT just a watt. We measure power under steady state conditions, using a resistive load. Music is anything but steady state, and despite Bud Fried's raison d'etre, loudspeakers are not at all resistive loads. Not to complicate matters too much, but loudpeaker (bass alignment AND crossover design) topology will greatly impact how much power an amplifier can put into it.

That said, all things equal, I've seen so many less powerful amplifiers put out more seat of the pants power than higher power amplifiers, and have come to the conclusion that the audio industry has not properly discovered how to measure actual / real-world / musical power. The most obvious example I can list is an 11 wpc push-pull 2A3 tube amplifier that thoroughly out-muscled the same manufacturer's 120 wpc hybrid tube / solid state product.

Compare Alan Watts to Reggie Watts, and you will find that one continues to produce sound and the other does not.

A watt is a unit of quantity and all are the same, but each case is different regarding the characteristics of each watt produced and how it sounds. Same amp can perform radically different with different speakers.

So saying a watt is a watt is true in theory in terms of how much power is generated, but in practice, all watts will be different sounding case by case. So there is certainly credence in saying that and value in knowing it, but alone it still determines only one piece of the puzzle in practice.

Are you trying to tell tube/solid state amps measure RMS differently?

10-12-15: Frogman
I will let the more technically astute than I debate the technical side of this
issue, but experiences with both tube and ss amps have shown me that
there is much more going on than "a watt is a watt", or wether
the amp in question is driven into clipping and how it reacts to being driven
into clipping.

I'm afraid that a watt is a watt & it is the distortion characteristic of a tube amp vs. that of a s.s. amp that appears to give the listener the impression that a tube watt is more powerful than a s.s. watt. It is not.

I bought my first pair of Stax F-81 electrostats back in early '90's when my
system included a NYAL Moscode 600.

this makes sense - an amp that is good for driving dynamic cone type loudspeakers (Thiel) & magnetic planners (Magnepan) cannot be assumed to be good enough to drive an electrostatic speaker. Electrostatic speakers are effectively a large capacitor to the power amp. This model of a capacitor for an electrostatic speaker comes from the fact that you have a stator on either side of the rotor/energized thin film that effectively creates 2 parallel plates of a capacitor where one is the top-plate & the other the bottom plate. Both stators create either the top-plate or the bottom plate. If the electrostatic loudspeaker looks like a large capacitor to the power amp, it also means that the impedance of such a speaker follows a 1/f profile i.e. speaker impedance is very high at low freq & very low at high freq. Just the opposite of a cone type speaker or even a magnetic planar. Since the electrostatic speaker's impedance is very high in the bass region, guess what?, the power amp has to pump current into a high impedance at the bass freq. Any s.s. or hybrid amp (which acts like a constant voltage source) will reduce its output with increasing speaker impedance. No wonder your NYAL Moscode 600 sounded horrible with an electrostatic & it was totally expected. A tube did much better because most tube amps act like constant power sources constantly adjusting their output current & output voltage to keep output power constant as the speaker impedance changes. This also means that a tube amp can give you relatively constant power (20% variation can be expected) over the 20Hz-20KHz range while a s.s. & hybrid amp will decrease its power into a higher impedance speaker load. it is no wonder that the Dynaco outdid your NYAL hybrid amp. Totally expected.
You have to be very careful which amp you connect to an electrostatic speaker due to the speaker looking like a capacitor to the power amp. Most power amps oscillate & self-destruct when they have to drive large capacitive loads.
It is no coincidence that SoundLab customers use tube amps almost exclusively (I think a lot of them use Atma-sphere amps) & that Sander Sound Labs makes a special s.s. amp for electrostatic speakers.

10-11-15: Bifwynne
Thanks Bombaywalla. I really enjoyed reading the 1983 Atkinson and Messenger ......

welcome Bifwynne.
yeah, I figured it might be difficult to understand what was written in that patent application not only because of the material of the subject but also due to the fact that it was written in technical legal-ese that patent attorneys love!
I believe that the basic premise of the partial cathode-coupling in the ARC output stage is to reduce output distortion while reducing impedance in the output stage circuit at the same time. This technique also allowed the ARC amps to have a "complex load driving ability" (written in his patent application) which in plain English means that they could drive tough speaker loads (the types that exhibit crazy phase angles that flip between capacitive & inductive over the audio spectrum).

Ralph,
thanks for pointing this out - you're right feedback & output power are not related. I should have written that sentence differently pointing out that output power depends on the topology of the tube - triode vs. ultra-linear vs. pentode with the output power going progressively higher as one moves from triode to pentode.

Tube amps using ultra-linear mode of operation use a lot of global negative feedback to generate a higher wattage.

Bombaywalla, taken either in or out of context its hard to take this comment seriously. I've modified many Dyna ST-70s where the only feedback was the ultralinear operation itself and the amps worked fine. But the real problem with this statement is you can't apply feedback to get greater power. If the amp is capable of greater power is something entirely independent of feedback.

"Output stage coupling is a combination of “ultralinear” and Audio Research’s patented “partially cathode-coupled” topology, which is superior to conventional pentode or triode operation."

Its funny to me that they got a patent on that since Leslie amplifiers (of Hammond organ fame) had used the same technique in the 1950s.

I'm not even treading into the realm of "is 14dB too much global negative feedback?"!! ;-)

14 db is an 'inbetween' figure which normally leads to *increased* distortions of some harmonics, but as others have said the ARCs use other forms of feedback as well- ultralinear being one, and the cathode cross-coupling being another. If neither technique were used 14 db would very likely be problematic.

tube amps have no bass hence it's power mostly applied towards mids and highs while the solid state amps go full range.

As prediceted, I take umbridge with this remark as it really is simply false. Our amps make full power to 1 or 2Hz depending on the model. But what is really going on here is that most box speakers don't have decreasing impedance as frequency goes down; while many have dual woofers arrays, they will also have an impedance peak that is part of the resonance of the drivers in the box. So if you have a 4 ohm tap you can likely make that work on a tube amp.

More importantly is the question of whether that's even a good idea, as every amplifier technology known has audibly higher distortion when driving lower impedances. I've yet to encounter a good reason for increasing amplifier distortion if sound quality is the goal!!

well some of them isn't most of them. it's nature of tubes and output transformers. they're weak when impedance of speaker goes down on low frequencies.

This statement is false. First- tubes are not frequency-dependent devices and often have bandwidth far in excess of most semiconductors. The design of the output transformer is the real variable- its no problem designing one that will do quite well in the bass. The trick is doing that and getting the highs right at the same time.

Yes, a watt is a watt, but, the way most tube manufacturers measure their gear, a tube watt is, arguably, LESS than a solid state watt because tube manufacturers typically spec the output at a much higher allowed distortion level.

This statement is misleading. The way both solid state and tube amplifier manufacturers do it is with **the onset of clipping**. Many tube amplifiers will have higher THD at the onset of clipping than solid state amps might. FWIW that THD is usually of lower-ordered harmonics that the ear finds to be musical, whereas solid state amps are far more likely to have much higher levels of the 7th harmonic, which the ear finds to be really objectionable. Hence the tube/transistor debate over half a century after tubes were declared 'obsolete' and yet the tubes are still here.

And if you are thinking 'well all we have to do is build a solid state amp that doesn't make any 7th and it will sound as smooth as tubes' you would be nearly correct, so have at it!! The best designers out there (Nelson Pass, John Curl, Charlie Hanson) are all aware of this fact and have been for a long time. Its no coincidence that they have designed some of the best sounding transistor amps made.

I will let the more technically astute than I debate the technical side of this
issue, but experiences with both tube and ss amps have shown me that
there is much more going on than "a watt is a watt", or wether
the amp in question is driven into clipping and how it reacts to being driven
into clipping. One experience in particular proved this to my ears:

I bought my first pair of Stax F-81 electrostats back in early '90's when my
system included a NYAL Moscode 600. The Stax are known for being a
brutal load for most amps and very inefficient. The Moscode is a well
regarded hybrid amplifier which delivers 300 ss watts per side and which
had, up until then, done a good job of driving a variety of speakers
including Thiels, Magnepans, and others; and sounded good doing so.
After looking for the Stax (which I considered, and still do, to have the best
midrange I have ever heard) for quite some time I finally found a pair. I still
remember well the disappointment I felt when I set up the Stax and heard
how poor the sound was. There was plenty of volume (as much as the
Stax can provide) without any obvious distortion, but the sound was thin,
lifeless and grainy. This led to trying several different speaker cables and a
couple of different preamps (?). No significant improvement; bad sound.

I was ready to sell the Stax when, coincidentally, I visited my local
appliance repair shop to buy some vacuum cleaner bags and I noticed on a
back shelf a repaired Dynaco ST-70 which had not been picked up for
months. The shop sold it to me for the cost of the repair ($85). I brought it
home and, just for the hell of it, replaced the Moscode with it; thinking, no
way! To say that I was amazed at the result would be a huge
understatement. The Dynaco's 35 tube watts did not play as loud as the
Moscode's 300 ss watts (duh), but it sounded beautiful (by comparison),
fairly refined, dimensional, and within a more narrow dynamic range it let
the music flow and move as it should; the music finally sounded alive. I
bought another pair of the Stax more recently and drive them to great effect
with Manley 200 tube monos. Ever since that first Stax/Dynaco experience
I have owned only two ss amps (BEL and Levinson) and compared to the
Manley 200 mono's which I currently use the differences between the ss vs
the tube amps are very similar to the differences between the Moscode and
the Dynaco; although at a much higher level of overall fidelity.

well some of them isn't most of them. it's nature of tubes and output transformers. they're weak when impedance of speaker goes down on low frequencies.

Yes, a watt is a watt, but, the way most tube manufacturers measure their gear, a tube watt is, arguably, LESS than a solid state watt because tube manufacturers typically spec the output at a much higher allowed distortion level.

BUT, measurements notwithstanding, I actually have heard many demonstrations where a low-powered tube amp is perceived to sound louder than a much higher powered solid state amp. In such demonstrations, neither amp was pushed to obvious clipping. At modest or low sound levels, I think that many solid state amps sound dead and lifeless. To get them to sound livelier, one tends to push the volume. When heard side by side at the same volume, one is often struck by how much better a well designed tube amp sounds compared to a comparable solid state amp, and because so many audiophiles conflate sound characteristics down to some simple measurement, like power and distortion, they concluded that the better sounding amp is more powerful.

well some of them isn't most of them. it's nature of tubes and output transformers. they're weak when impedance of speaker goes down on low frequencies.

Tim deParavicini (EAR) has said that he can tell what an amp's bass will sound like by looking at the power transformer. For bass, the larger the better. Unfortunately, the larger it is, the worse it is (all else being equal) at high frequencies. An amp designer has to balance the two against each other and find the best compromise.

Thanks Bombaywalla. I really enjoyed reading the 1983 Atkinson and Messenger interview of Mr. Johnson. It was like taking a trip back in time.

IMO, since that time, ARC has made many significant refinements to their technology that have taken its product line many level beyond where ARC was holding back in 1983.

Wish I could say I understood the patent app stuff ... but I didn't. I'm not a EE.

I think Mr. Johnson wisely commented that trying to judge the quality of an amp by throwing around cold stats like slew rate, phase angle shift, bandwidth, and so forth is a fool's errand. It made perfect sense to me when he said many technical factors are taken into consideration when designing an amp that sounds good. To even think that a perfect amp can be viewed as "gain on a wire" is an absurdity.

Thanks again.

Some actually think that only tubes can make the bass sound right. Or at the vey least something like Lamm hybrids.
This is probably a slight exaggeration and a matter of taste and speaker choice, but I can see a strong point.

10-10-15: Bifwynne
Thanks Bombaywalla ... but what is the purpose of partial cathode coupling. Is it a form of local negative feedback?? Does it ameliorate some of the adverse affects associated with NFB that Ralph has written about, e.g., TIM distortion that raises the level of odd ordered harmonics??

ARC has used this type of topology for many years.

Search & you shall find!!! (this is note to self)
Look what I found:
* an article in Stereophile where the late William Z Johnson was interviewed by John Atkinson. WZJ talks a wee bit about partial cathode coupling & gives the credit to QUAD as the initial inventor since QUAD used it in their Quad30 amp some 30 yrs prior to that interview. See para #3 from the top:
http://cdn.stereophile.com/content/william-zane-johnson-1926%E2%80%932011#mX6ilT78dampubmG.97

* next, I found William Zane Johnson's patent application on the partial cathode coupling (must have been an enhancement of the Quad's version?). This is publication US3566236 A that was filed in 1968 & published in 1971. Yeah, you are correct - ARC has been using this for a long time in all their products.
http://www.google.com/patents/US3566236

You can read this stuff but here is the crux (cut & pasted from the patent application)
"Still another object of the invention is to provide an improved amplifier output stage coupling with partial cathode coupling, while maintaining classic tetrode operating parameters with substantially the same efficiency and drive requirements."

hope this provides some more insight, Bifwynne.

"Tube amps have no bass" seemingly doesn't address the countless tube bass amps like the amazing Ampeg SVT that's been around since 1969…Mesa made an astonishing sounding tube head in 1983 or something that a dude in my band back then used. Also note that tube hifi watts have been proven in many scientific studies using sine waves, pink noise, grunge music, various speaker loads, various bong loads, yo mama, and luddite resistors to illustrate the fact that tube watts are more fun. My Jolida 502p sounds best using the 4 ohm tap…it's a fave. Another note about tubes…the Woodstock ('69) sound was powered by piles of McIntosh tube amps under the stage, a fact I recently discovered that makes me happy.

Beavis ... there are ... many other factors. As Kijanki posted, music is more than listening to sine waves.

@Kijanki ... thanks for your informative response. You wrote:

"Damping factor of 14 is OK. 8ohm speaker's impedance is mostly resistive. Assuming, that it is approx 6 ohm it limits effective DF to 1.33 . Amps DF of 14 will make it worse only by 9.5% - irrelevant."

In my case, I drive my speakers, which have roller coaster impedance and phase angle plots, off the 4 ohm taps. To my ears, those taps sound "overall" the best. While it may result in non-optimal impedance matching at higher frequencies, I surmise that the power demands place on the amp are not that significant at such frequencies. Further, as mentioned above, the amp has a very robust power supply. Hence, I doubt that the mismatch is resulting in considerable distortion.

Kinjank, you also write that:

"There is a lot of local NFB in almost every amp. Any resistance in cathode is a form of NFB. Global NFB doesn't have to create TIM if it is applied within certain limits. It improves pretty much everything - bandwidth, output impedance, THD & IMD. Great sounding amp with small amount of NFB requires great design and quality components. Unfortunately it is cheaper to achieve the same using cheaper design and excessive amount of NFB hence creating overshoots (odd harmonics in frequency domain) and unpleasant bright sound."

I surmised the same as posted above. I wrote that ARC said the use of NFB is part of an overall engineering solution that balances many variables.

Thanks again.
BIF

Wpc is wpc. I think there are other factors at work here to account for differences (as Mapman implies).

I believe Atmasphere mentioned once particular B&W speakers consisting of lower efficiency 4 ohm bass speakers with higher efficiency 8 ohm midrange and tweeter. Speaker has great bass when amplifier doubles current on 4 ohm speaker, but not so great when driven by tube amp that doesn't do that.

I think bass response with either tubes or ss, might depend more on how the speakers used with them handle damping. With that said; in my experience, dynamic speakers with lower impedance numbers mated with ss amps that can double down typically have superior bass response.

Completely agree with everyone about the bass - if you get the right tubes, bass is certainly never lacking. Actually, with tubes, you have more flexibility to tailor it to your tastes. Certainly more tubes with have different sonic characteristics but if you want bass, I would not suggest shying away from bass. It's all preference at the end of the day.

So sorry goodbye .

10-10-15: Czarivey
"tube amps have no bass hence it's power mostly applied towards mids and highs while the solid state amps go full range."

You couldn't be more mistaken! In fact, some tube amplifiers are known for their excellent LF response.

Music requires very little average power, unless one listens to sine waves. It is possible that two of 50W amplifiers can be designed with very different headrooms. One might have small headroom being, for instance class A amp while the oder has huge headroom with average power limited only by size of power supply and heatsinks. Amp with higher headroom might appear much louder without distortion.

14dB is rather shallow feedback.

Damping factor of 14 is OK. 8ohm speaker's impedance is mostly resistive. Assuming, that it is approx 6 ohm it limits effective DF to 1.33 . Amps DF of 14 will make it worse only by 9.5% - irrelevant.

There is a lot of local NFB in almost every amp. Any resistance in cathode is a form of NFB. Global NFB doesn't have to create TIM if it is applied within certain limits. It improves pretty much everything - bandwidth, output impedance, THD & IMD. Great sounding amp with small amount of NFB requires great design and quality components. Unfortunately it is cheaper to achieve the same using cheaper design and excessive amount of NFB hence creating overshoots (odd harmonics in frequency domain) and unpleasant bright sound.

Czarivey ... I think your post slaps quite a bit of paint with a very wide brush.

My ARC Ref 150 SE has a relatively low output impedance, resulting in a damping factor of 14 (assumes an 8 ohm load; would be half if the load is 4 ohms). That ain't bad for the bass. In addition, it has a 1040 joule power supply, which helps the amp to muscle its way through demanding loads where serious current (and correlatively power) is needed.

And based on my own ears, I do not concur that my tube amp "has no bass" or that its power is "mostly applied toward mids and highs." I surmise that Ralph (Atmasphere) would take similar exception about his highly regarded amps.

Thanks Bombaywalla ... but what is the purpose of partial cathode coupling. Is it a form of local negative feedback?? Does it ameliorate some of the adverse affects associated with NFB that Ralph has written about, e.g., TIM distortion that raises the level of odd ordered harmonics??

ARC has used this type of topology for many years.

tube amps have no bass hence it's power mostly applied towards mids and highs while the solid state amps go full range. 70w of tube amp power will scream out louder on mids and highs, while same of solid state will cover full range and will have substantially lower volume vs tube. otherwise power is just product of voltage and current not just watts.

10-10-15: Djfst
Tried it again and the 8ohm taps definitely sound better all day long compared to the 4ohm taps. Who knows?

there's a good explanation for it as stated in my earlier post. I guess you missed reading it??

Bifwynne,
yeah LOL! ;-)

@Bombaywalla ... do you understand the term "[o]utput stage coupling is a combination of “ultralinear” and Audio Research’s patented “partially cathode-coupled” topology ..."?? I have a rough understanding of ultralinear .... No clue about "partially cathode-coupled” topology. Any idea??
Bifwynne

well, this is a serious case of the blind leading the lame but I'll give it a shot. I hope that somebody who knows more steps in....
'ultra-linear' is a global negative feedback technique where, in a tetrode (4 active electrodes) or a pentode (5 active electrodes), some part of the output (usually present on the anode or plate) is fed back to one of the screen grids of the tube. The amount of output signal fedback depends on the tube type (KT88, 6550, 6V6, etc). David Hafler has patent on this where he fed back 43% of the # of primary xformer turns to the screen grid to linearize his amplifier. What this global negative feedback did was reduce output distortion, reduce output impedance & raise the output power to near-pentode levels.

'cathode-coupled' is a tube amplifier topology where the input tube is cathode coupled to the next gain stage tube. here are some pix for you to look at (you can drag your mouse over the pix without any harm. Click on the pix at your own risk!)
http://www.bing.com/images/search?q=cathode+coupled+amplifier&qpvt=cathode+coupled+amplifier&qpvt=cathode+coupled+amplifier&FORM=IGRE
the 1st pix itself should be good enough to allow you to visualize what I wrote.
In the pix the 2 tubes are strongly coupled as there is a deliberate connection between the 2 cathodes. The issue with this amp is that the cathode of the input tube is at a very different voltage than the cathode of the 2nd tube. So, this amplifier becomes very sensitive to power supply variations & noise. Thus, I'm *assuming* (you know this goes!! ;-) )that ARC has a patented technology where they partially couple the 2nd tube to the 1st tube so that they can make the cathodes of both tubes operate at nearly the same voltage while still transferring the input signal to the 2nd tube to amplify it.
I could be out on a limb here.....

10-10-15: Ebm
Bomb you may suck wind DUDE!!

Dude, *dont* get personal here w/ the name calling. No one's doing it to you so you better return the favour. I wont say it again....

Tried it again and the 8ohm taps definitely sound better all day long compared to the 4ohm taps. Who knows?

@Bombaywalla ... "I'm not even treading into the realm of "is 14dB too much global negative feedback?"!! ;-)" Scardy-cat! :) LOL

Re NFB: FWIW, many years ago, I recall speaking with either Lenard or Kal (ARC customer service tech rep; Len has since retired) ... I can't remember which one now ... and I asked about ARC's use of NFB in their amps. The answer was that many electrical variables are taken into consideration when designing an amp. So ... ARC's end product is the result of many engineering compromises that are made to achieve an optimal result. No perfect answers or solutions to an engineering problem that entails making trade-offs.

@Bombaywalla ... do you understand the term "[o]utput stage coupling is a combination of “ultralinear” and Audio Research’s patented “partially cathode-coupled” topology ..."?? I have a rough understanding of ultralinear .... No clue about "partially cathode-coupled” topology. Any idea??

All the "techno" BS aside, I recommend that the owner of the SFs and the Prima Luna gear should use the taps that sound best to his ears.
Bifwynne

I agree.
In that post, given that the OP declared the 8 ohm tap was the best for his SF speakers, I tried to see if I could co-related his decision to the impedance & phase plots I found online.
So, it was the other way around.
I thought that was clear from my post since I quoted the OP before starting my reply but I guess not. Sorry if I lead you & others into thinking that one could read graphs & make a final decision which amp tap would be the best.

I'm not even treading into the realm of "is 14dB too much global negative feedback?"!! ;-)

Bombaywalla, I just caught your post re the Soundstage measurements of the SF speaker. I surmise that you would agree with the general view of many that when deciding on a tube amp's output taps to use for a particular speaker, that one should use the taps that sound best to that person's ears.

That said, if I was asked to guess what would be the best output tap selection for the SF speakers based JUST on graphs and NOT ears, I might have guessed the 4 ohm taps would be optimal. Here's why.

Btw, I assume the SFs were voiced to be driven by a SS amp. I also tracked down a J Atkinson bench test report of the Prima Luna HP amp. JA's output impedance measurements are pretty typical: 4 ohm taps (between 1.1 and 1.3 ohms); 8 ohm taps (about double the 4 ohm taps).

First off, the SFs are moderately sensitive. Soundstage reports 88.3 db. That bodes well when worrying about driving speakers with rough impedance and phase angle plots in the bass frequencies.

Second point. As JA reports, the amp's output voltage will vary with speaker load because of the amp's "high'ish" output impedance. Presumably, the amp's output voltage should vary less off the 4 ohm taps because output impedance is half that of the 8 ohm taps.

Third point. The Soundstage report shows that SF's FR drops off pretty sharply below 70 or 80 Hz. I suspect that the owner may be using a self powered subwoofer. If so, my reactions are even more on point.

Fourth point. The Soundstage report also shows that the SF's impedance ranges between 3 and 5 ohms between 50 and 100 Hz. In addition, the SFs are mildly capacitive (neg. 23 degrees or so) in that frequency range. Could be worse. Further, speaker impedance stays at 5 ohms or below up to 1000 Hz.

Conclusion. All in, my guess ... based JUST on the numbers ... is that the 4 ohm taps would yield a cleaner sound with less frequency response variation because of the amp's output impedance off the 4 ohm taps. In addition, the back impedance off the primary taps of the output trannies would probably be a better impedance match for the tubes because the amp is asked to deliver most of its power output in the 3 to 5 ohm range.

All the "techno" BS aside, I recommend that the owner of the SFs and the Prima Luna gear should use the taps that sound best to his ears.

10-10-15: Inna
I don't remember where but once I read that someone was comparing the sound of 100 wt Gryphon with 600 wt Krell, both transistor amps. Same speakers,forgot what that was. Besides sounding a lot better in every respect, Gryphon also sounded more powerful. Dynamic speakers, this I remember.
What are the possible explanations?

Impossible to tell without knowing how the testing/comparison was made... My guess is they did not level match, or make quick switches between components - the two main factors that our senses need, in order to make a proper evaluation...

Bombaywalla, as you probably know, my amp is the ARC Ref 150 SE. I mention this because ARC designed my amp, and indeed, most of its tube amps, using NFB. In the case of the Ref 150 SE, I believe the NFB stat is 14 db.

Ralph has written quite a bit about the negative trade-offs of using NFB. Do you consider 14db to be "a lot of global negative feedback"??

FWIW, ARC describes its Ref 150 circuit topology as follows:

"Output stage coupling is a combination of “ultralinear” and Audio Research’s patented “partially cathode-coupled” topology, which is superior to conventional pentode or triode operation."

Whatever that means. :) LOL

10-09-15: Djfst
High end audio is definitely tricky. For example, I'm running a Primaluna Dialogue HP Integrated Amp with EL34 tubes to Sonus Faber Olympica III Speakers which are 91db efficient and are 4ohms rated according to specs. The Primaluna has 4ohm taps and 8ohm taps, and the 4ohm speakers sound far better on the 8ohm taps.

I found your speaker measurements here:
http://www.soundstagenetwork.com/index.php?option=com_content&view=article&id=1285:nrc-measurements-sonus-faber-olympica-iii-loudspeakers&catid=77:loudspeaker-measurements&Itemid=153
if you scroll down & look at the 2 plots "impedance magnitude variation" & "phase angle" you can see that this speaker has capacitive phase angles in the bass region to, oh, ~150Hz. The impedance is complex i.e. has a x+jy sort of form. So, the real part of the speaker impedance into the power amp is delivering current is not 4 Ohms as printed on the back of the speaker but less than 4 Ohms due to the capacitive (reactive) phase angle. So, any power amp would be asked to deliver more current than originally thought (since you are thinking it's a 4 Ohm speaker) & this can tax the power amp.

When you connect your SF to the 8 Ohms tap, the power amp is being asked to deliver more current into the lower (4 Ohms) speaker impedance compared to when you have it connected to the 4 Ohms tap. The higher current delivered yields a better bass response.
The plots show the speaker is mostly inductive in the 150Hz-2.5KHz. So, when high voltages are present in the mid-bass-to-mid-range little current is asked from the power amp due to this inductive impedance nature of this speaker in this region (the math omitted deliberately). And, in the high-freq even tho the speaker become capacitive there is not much energy in those frequencies so the power amp is not taxed much.
So, it seems to make sense that this SF speaker would sound better on the 8 Ohms tap.

The manufacturer recommends 50-180 watts and received wisdom seems to be that the speakers respond better to solid state watts (what does that mean)?
Mikey8811

I've not looked at the Vienna Acoustics Kiss speaker impedance & phase plots but if this is the wisdom that has been imparted to you then it would mean that the VA Kiss has some wild impedances & phase angles in the bass region where the actual impedance is quite low where a tube amp would have difficulty sourcing large amounts of current to have a controlled bass response. In such a case a s.s. amp would do better esp. if it can double down for each halving of the load impedance. This would be an expensive s.s. amp since it would have a very robust power supply. Heavy chassis, heavy power xformer, large heatsinks, high cost but it would give you a tight bass response while a tube amp would give you flabby bass response unless you spent $$$ to get a large tube amp.

An interesting phenomena I noticed from decades of guitar amps is the tactile "snap" of tubes. Many geezers like me tried out various SS guitar amps over the years and regardless of design most of us return to the warm land of tubes (some cleaner playing jazz dudes still like Polytone amps, and bass players often prefer the extreme wattage of SS like my Class D Ampeg)…the seemingly ballsier "clean" tone (all about the aforementioned tube vs. SS harmonics) is where a lot of the mojo is revealed, as that means the underlying grease will rise to the surface. Modeling amps…meh…my hifi tube amp also "seems" to be ballsier at other than extreme levels, and those extreme levels are generally accompanied by inebriated dancing madness so they might not count.