Tube Watts vs. Solid State Watts - Any credence?


I've heard numerous times that Tube watts are not the same as Solid State watts when it comes to amps running speakers. For example, a 70 watt tube amp provides more power than a 140 watt solid state amp. Is there any credence to this or just sales talk and misguided listeners? If so, how could this be? One reason I ask is a lot of speakers recommend 50 - 300 watts of amplification but many stores have 35 watt tube amps or 50 watts tube amps running them. More power is usually better to run speakers, so why am I always hearing this stuff about a tube watt is greater than a solid state watt?
djfst
The difference in perceived loudness between 70W and 140W amplifier is only 22%. Tube amps, when overdriven tend to produce "soft clipping" (softer/gradual overdive transition), while SS amp just chops off the peaks producing a lot of unpleasant sounding odd order harmonics.

Some SS amp were designed to soft clip, including my class D Rowland 102 (and some NADs) but most of them don't. Overdriving SS amp is not only much more noticeable, but can also damage tweeters because of excessive energy in high frequency harmonics.
Most tube amps soft clip so effects of not enough power are
not as unpleasant. Other than that, a watt is a watt.
What's different is how far a watt can be made to go. Soft
clipping helps. Limited bass extension does as well. These
are things that can help make a speaker seem more
"efficient". You need exponentially more power to deliver
a lower frequency at a certain volume level. That's
physics....basically how things work no matter what anyone
might claim. That's why a table radio with just a few
watts and limited bass extension might sound clean within
its limitations at a louder volume.
Kijanki is very right---a hard clipping amp is dangerous to the health of your tweeters. I had a low-powered SS amp burn out a dome tweeter in the blink of an eye when it clipped, and the volume was pretty low.
For example, a 70 watt tube amp provides more power than a 140 watt solid state amp. Is there any credence to this or just sales talk and misguided listeners?
it's just sales talk & misguided listeners. No credence to this statement. As Mapman & many others before have written: a watt is a watt tube or solid-state.
Ralph Karsten of Atma-sphere has written a paper on power amplifier paradigms:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php
The effect has to do with the distortion of the amplifier.

Since a transistor amp clips harshly, its obvious when it happens. A tube amp has a soft clipping characteristic (which can be modified somewhat depending on how feedback is used with the amp). So even though the amp is overloading, as it enters clipping it won't **sound** like it- and as you push it harder, the higher ordered harmonics become more abundant. Since the human ear uses those harmonics to figure out how loud a sound is, their presence will make the amp sound louder than it really is. This works until the clipping is so severe that the amp starts to break up and makes hard clipping like a transistor amp.

If you place a sound pressure level meter in the room, the phenomena will be revealed, which is to say that watts is watts.
So a followup question - if 2 amps both have 140 watts, why ever buy the more expensive amp if you are paying for wattage? Most have indicated quantity of wattage, but can the quality of wattage be better in some amps than others? Secondly, how can low powered tube amps sound so much better with some speakers that have amp recommendations much higher? Very few tubes get over 100 watts, yet are very prevalent even on some inefficient speakers. Any thoughts? Just trying to learn more.
I am using a pair of 40w EL34 monos driving a pair of 87dB efficient bookshelves full range with a sub crossed over low just to augment what my speakers can't reproduce and it rocks....plenty of power in my 15 x 17 x 9 room

I had previously used a McCormack DNA 125 which was a great amp no doubt but I don't think I even came close to it's full power even at louder volumes
I was just about to ask the same question.

I moved from Dynaudio's at 86 dB to Vienna Acoustics Kiss at 89 dB, both into 4 ohms and am looking to change my amp from a Krell FPB 200c to a tube amp - either a Conrad Johnson LP 125 sa or a Cary CAD 120 S Mk 2 both at 120 plus watts vs the high current Krell which doubles down into 400 watts.

Will I get sufficient watts from either of the tube designs? 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)?
Do

That's like saying why ever buy the more expensive tv if they are both 52 inch. Or the more expensive car if both have same horsepower engine.

With technology no single spec ever tells the whole story. In fact specs as a whole cannot. No two amps are exactly the same. There many differences some subtle and some profound. Quality being just one general category. Same as with any complex piece of technology.
It might actually be true that: "...a 70 Watt tube amp provides more power than a 140 Watt solid stat amp." if used on speakers with an impedance of over 16 Ohms.
But, selecting an amp involves more than just this issue, there are other variables to consider.
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 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?
Uh - a watt is a watt ...it is a unit of measure there is no difference by virtue of the device that produces it.

Now frequency characteristics, load interactions and clipping or distortion characteristics- that is different.
A practical question out of all of this is power tube choice in the same amp. Currently am running eight EL34's tubes in a primaluna HP integrated amp to Sonus Faber Olympica III speakers. The Sonus Fabers are 91db, 4 ohms, and recommended amplification of 50-300 watts.

Tube choices in the amp are as follows:

ULTRALINEAR MODE:
85 watts x 2 (KT120)
73 watts x 2 (KT88)
70 watts x 2 (EL34)

TRIODE MODE:
45 watts (KT120)
42 watts (KT88)
40 watts (EL34)

I know tubes have there own characteristics, but besides that, from a power standpoint, would it be better to go to KT120s to get more power to my speakers? Would the difference in wattage moving from EL34s to KT120s (15 watts increase in ultra linear, 5 watt increase in triode), be beneficial or even perceptible to my ears? Would this be better for the performance of the speaker, or at least the health of the speaker?
It's all about the sound of your amp not the juice. i just went back to a tubed separates set up, trading in a 3 month old McIntosh MAC6700 solid state receiver for an MC275 tube amplifier. Since I rarely drove the receiver past 30 watts, and it was "voiced" to emulate the McIntosh tube "sound" I figured why use solid state in the first place?
(MC275, Audio Research LS3 SS preamp, Von Schweikert VR 3 speakers)
The 275 sounds true, with an openness the solid state appeared to muffle. I don't believe I have ever clipped any one of my amps over the years since I listen at fairly moderate levels. If I return to solid state I will go back to Pass Labs
Wow thats a tough one i was up all night thinking about that!!
"Currently am running eight EL34's tubes in a primaluna HP integrated amp to Sonus Faber Olympica III speakers"

This is a coincidence. The guy who bought the Krell FPB 200c from me is using Sonus Faber Cremona (the originals) an erstwhile predecessor of your Olympica 3's with likely the same Prima Luna integrated. He muttered something about 40 watts. He said there wasn't enough juice to push the sound out of his speakers. A few hours with the Krell in his setup, he calls me up and thanks me for the best thing he has ever done to his speakers as they really sang like they never did before. I believe Franco Serblin voiced the Stardivari's with older FPB Krells. I understand the new Sonus Faber's are more sensitive. I heard them with ARC Ref electronics and they left me cold compared to the older Sonus Faber sound. I guess the last Sonus Fabers I liked were the Stradivari's and the Guarneri Memento's (not heard the Evo's)

Not doubting what you hear in your system but I guess different strokes for different folks.
10-09-15: Djfst
So a followup question - if 2 amps both have 140 watts, why ever buy the more expensive amp if you are paying for wattage? Most have indicated quantity of wattage, but can the quality of wattage be better in some amps than others?
The more expensive amp has a more robust power supply which most probably allows that power amp to double down with each halving of the load impedance. This means that the more expensive amp also has more current delivery capability which in turn means that it can power hard-to-drive speakers far more easily than the less expensive amp.
This is based on the assumption the you are not comparing 2 very disparate brands i.e. both amps are of brands that have similar reputation & command similar prices in the marketplace.

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?
the above is an explanation for your question Inna. In this specific case the Gryphon doubles down all the way to 1 Ohms. One of the rather rare s.s. amps in the market & very expensive. My friend used to own the Antilleon Signature which was a 100W/ch amp but was 800W/ch into 1 Ohms. Beuatifully built amp - like a tank & almost no speaker could ruffle its feathers (but don't hook an ESL to this amp as it was not meant to drive an ESL).
Current delivery into a speaker load is very important esp. when it comes to hard-to-drive speakers (which are characterized by having wild impedance & phase swings).
Secondly, how can low powered tube amps sound so much better with some speakers that have amp recommendations much higher? Very few tubes get over 100 watts, yet are very prevalent even on some inefficient speakers. Any thoughts? Just trying to learn more.
Djfst
Atmasphere explained this quite clearly in his post above. Did you miss it?
it's the distortion characteristic of tubes amps - when they're overdriven a little the distortion is more pleasing to the ear than a s.s amp that is overdriven the same amount.

I know tubes have there own characteristics, but besides that, from a power standpoint, would it be better to go to KT120s to get more power to my speakers? Would the difference in wattage moving from EL34s to KT120s (15 watts increase in ultra linear, 5 watt increase in triode), be beneficial or even perceptible to my ears? Would this be better for the performance of the speaker, or at least the health of the speaker?
Djfst
in the end it really depends on what kind of sonics you prefer.
Tube amps using ultra-linear mode of operation use a lot of global negative feedback to generate a higher wattage. These sort of amps have a very robust sound & trend to sound more like s.s. amps. And, behaviour-wise they also behave like constant-voltage sources (which most s.s. amps can be characterized). I've found that ultra-linear tube amps had more bass punch but their mids & highs do not have the delicacy that is often associated with tubes.
Operating an EL34 in triode mode would be operating this tube in as linear a mode as is possible. Remember the EL34 is a pentode tube but in triode mode it operates like a tube with 3 units (sorry I could not think of a better word at this time!) - anode, cathode & 1 screen. You get much less wattage (as you have stated) but you get a beautiful tube sound that trends towards the type you hear from more traditional triode tubes such as 300B, 2A3, PX25, etc. At one point in time i used to own an AES Sixpac that had 6 EL34 per chassis & output 60W/ch in triode mode. It was a very addictive & beautiful sound. That amp drove my then-speakers with plenty of power - didnt have the bombastic bass of a s.s. amp but i did not care as I got much more from that amp that compensated for a less bass response. When I changed my speakers that amp had to go. Personally, I'd go for linearity hence triode mode. But like I wrote before - you might like you music delivered differently.....
10-10-15: Ebm
Wow thats a tough one i was up all night thinking about that!!
EBM, I'd like a different snidy punch-line. I've laughed at this one one too many times! Search your sarcastic brain for a new snidy remark. Thanks.
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.
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.
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.
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
10-10-15: 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, 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.
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'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??
Post removed 
Tried it again and the 8ohm taps definitely sound better all day long compared to the 4ohm taps. Who knows?
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....
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.....
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.
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.
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.
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.
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.
So sorry goodbye .
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.
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.
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.
Wpc is wpc. I think there are other factors at work here to account for differences (as Mapman implies).
@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
Beavis ... there are ... many other factors. As Kijanki posted, music is more than listening to sine waves.
"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.
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.
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.
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.
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.
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.