Another 6550 vs. KT88, 90, et. al.?

Recent listening after having the tetrode/triode switches in my VTL 185's restored by the factory to functioning status (a previous owner removed them) has me reassessing the amps' tonal balance as presently configured (with Svet 6550C's, new last fall). While the tetrode setting I was confined to before the repair holds most of the 'high fidelity' aces (dynamics, space, separation, resolution, articulation, detail, extension, bass definition), my newfound ability to access the triode option has confirmed, and strengthened, my previous suspicions about the overall balance: A little spotlit in the lower treble, a little tight from the lower mid's down through the mid-bass.

The result is a tad hard and lean (for a tube amp), with a bit of glare above and a lack of plushness and body below. Triode corrects this balance toward the warm side, more neutral overall IMO (except at the frequency extremes), and also gives a smoother textural presentation with lowered grain, but at a cost in the other areas mentioned above. In general, the amps pass more info and get better speaker control in tetrode, but are purer and more natural through the heart of the audioband in triode, if also less present and more homogenized.

I know none of this is revolutionary news to many, and obviously questions of tonal balance will be highly system-dependent as well.* And in the big picture the 185's are the most revealing and least colored and distorting amps I've owned. But before plunking down for 12 more power tubes to experiment, and after having read through the archives, I wanted to bring this question back to the forum, especially in the hope that some VTL owners might shed more light on their power tube experiences.

*[Thiel CS2.2's/AZ Satori/VTL MB-185 Signatures (with 60's NOS Sylvania gold pin 6201/12AT7 inputs)/HT Magic Link One/Levinson 380S/HT ProSil/Theta Basic IIIa and Pearl/Shunyata Sidewinders on the amps, HT AC11's on the digital separates/API Power Wedge Ultra 116 on all]
what your describing in triode mode sounds to me like its a lack of power to me. Tetrode shouldn't appear to pass more information than triode also the presence should also be better in triode mode than in tetrode. Sounds to me the triode mode isn't enough power causing the resulting sonics your describing to me Is a stressed triode amplifier my 450 signature in triode sounded this way at very high levels it was running out of power which was solved by 750 reference is triode mode. Btw the harmonic technologies magic is not a good match with vtl in my experience see my review of the xlo signature 1.1 or signature 2.
Mejames, I too have thought about this factor, and it seems to me that it could well be part and parcel of why tetrode has the advantages it does. After all, one is not only switching from tetrode to triode operation inside the tubes with all other variables being held equal - doing this necessarily cuts the power rating in about half. A comparision to a similar amp run in triode, but with twice the number of output tubes to compensate, would be very interesting, but VTL's triode-only MB-250 and triode-switchable MB-450 are not this theoretical amp: They each purport to wring about twice the triode power from just 8 6550's as opposed to the 6 in the MB-185, and so must be running them quite differently. (I've never seen the 250 up close, but I know the 450's power supply and transformers, both power and output, are much larger than the 185's - and ought to be, at almost twice the price. The 450's weigh in 20lbs. heavier than my amps, but the 250's are only about the same - go figure).

When I do my listening tests between tetrode and triode modes, I take advantage of the Levinson preamp's programmable muting function to set it at only -3dB, which enables me to make level-matched comparisions without wasting time fumbling around with the volume control to compensate. (I've also found that I don't need to heed VTL's admonishment about powering down the amps before switching modes - everything is fine and calm switching on the fly, with no untoward transients at the speakers as long as I operate the switches quickly and decisively. Other VTL owners reading this account experiment at their own risk and on their own judgement when deviating from VTL's strict instructions regarding proper operation of this feature.)

My speakers - despite being Thiels, with all the power-sucking reputation that implies - are actually not a model that presents a particularly tough load to drive. The audible subtractions in dynamic contrast, for instance, when going from tetrode to triode are present even at fairly low volumes, and these amps are rated for circa 100wpc in triode mode, which based on my previous experiences with these speakers using amps rated at 45wpc and 70wpc should still be plenty of power. Another telling result is that the space around and between images is more explicitly defined and less amorphous and congested whenever tetrode is selected, again whatever the volume.

Nevertheless, it is true that the advatanges diplayed by tetrode mode seem a lot like what are often held to be - and held by VTL to be - the inherent advantages of higher power. The only advantages I hear in triode mode are in natural tonal balance (excepting the frequency extremes which are more rolled-off) and in ultimate textural smoothness. Other than those two (important) areas, in which tetrode is hardly intolerable itself, triode cedes to tetrode in every other way I can hear. Whether this finding is due solely to mode of operation, or is solely a pure power issue, or is a combination of the two I cannot say. My suspicions lie with the last scenario, but tilted somewhat toward toward the first, based both on past and low-volume experience, and on the fact that I haven't audibly approached the limits when running in triode at any volume, implying that I'm remaining well within the amps' comfort zone.

So my hope is to massage the amps' balance when run in tetrode. I figure I can look primarily at four areas (without changing speakers or signal cables - I do not hear a problem with the Magic One IC's vs. my previous Cardas Cross, but that topic is, of course, endless): Power tube substitutions, input tube substitutions, power cord substitutions (the Sidewinder's main virtues are a lot like tetrode's), and installing better AC lines and outlets. My mind for now is on the power tubes because the repair of the mode switches focuses my attention there. But this may not be the answer - Luke Manley says he prefers the Svet 6550C's, and Kevin at Upscale doesn't disagree. And VTL's low-negative-feedback design (6dB) mandates a highish output impedance (1.6 ohms) no matter what the tubes, which will cause some unpredictable tonal balance variability even with speakers, like mine, presenting a fairly flat impedance curve.

There is a fifth factor I'm strongly considering as well: Upgrading the coupling capacitors. I was planning on doing this anyway, and may still do it first, but what I've learned since getting the mode switches repaired is making me pause a bit before sinking any more money into the amps themselves. I'll probably get over it though - I was just shown that my amplification isn't quite as neutral as I may have supposed before the mode-switching was available to me. Big surprise. Nothing's perfect. It will ever be thus, and they still sound great, best I've had, even if right now I'm wishing for an unattainable combination of the best attributes of both triode and tetrode modes.
This is just my opinion, but I see no reason whatsoever to use the 6550 tube. The Legend Starlet integrated amplifier, a fabulous sounding product, designed around this tube(run pedal to the metal) excepted.

My complaints are basically everything you outlined Zaikesman. I'm not sure I wouldn't investigate a nice solid state amp over something running 6550. My money would be on the KT90 or KT88 every day of the week.

If I was after neutral sound, the Ei KT90 would be my choice, which is what I use in my Jadis. This tube is rugged, reliable, and very long lasting. Still going strong after 4 years. It is not a sweet tube, but offers a nice tone. It betters the 6550 in every area one would ever opt for that tube - bass, power, longevity, extension, neutrality.

As for the KT88, it is a sweeter sound, and sounds more like what most people consider classic tube. Will probably give them a go next in the Jadis. It's midrange is great, with a nice punch in the bass. And, I can't complain about the treble.

The JJ KT88 are my favorite, by far. After that, I guess I like the Svetlana. The JJ tubes, badged Teslovak, had a bad production run and ruined many a person's day a few years back, but recently a good friend of mine had a bad Svetlana do some serious damage to his Rogue amplifier/ Coincident speaker. So, who knows?
Trelja, I read carefully your comments on these tubes in the archived posts. In those threads there were some dissenters who seemed to be expressing basically the opposite view, but many seemed to agree with your take, although this variability could just be an accurate indicator of the importance of any particular circuit in question. This is the most part of why I started a new thread, in hopes of finding a VTL owner or two who've done this dance.

The way we are characterizing the 6550C sound here is reminiscent of the criticisms leveled at the earlier 6550 versions, which I only ever used in Marshall guitar amps. The rep of the "C" is that it is supposed to correct these tendencies. Then again, I've listened to VTL's small stereo EL-34 model in direct comparision to my old EL-34 equipped C-J and not heard any brightness, hardness, coolness, or lack of body in either, though neither possessed the overall fidelity of these mono's.

It is quite possible that the manufacturer's preference for the Svet 6550C is based largely on dependability. I found that I preferred the Svet's to the JJ's when it came to EL-34's (with Sovtek last), both in sound and dependability, but I note you reverse this preference for the KT-88's. I like the sound of EI small-signal tubes but have never heard a power tube of theirs. It would be great if I could find someone in the DC area who's got either KT-88's or 90's and who's curious to do a temporary swap if they've never run the 6550C's as an experiment, but I suppose that'd be a real long-shot.
Zaikesman, if you ever travel up the road to Philadelphi, you are welcome to give my 4 KT90 tubes a spin,
Trelja, thank you (I do get up to Philly from time to time, my bro lives there), but unfortunately each one of the monoblocks requires 6 power tubes. We could swap 4 of my 6550C's into your Jadis, but of course that wouldn't tell me what the KT-90's would sound like in mine...

Since I've taken the conversation (what there is of it - it seems like maybe I'm not going to find a VTL owner who's done the swaps) away from just a discussion of the various tube merits and into the realm of triode vs. tetrode in the VTL, I want to come clean about something I've glossed over in my response to Mejames. I mentioned that the only two aspects of the sound I thought were superior in triode mode were evenness of tonal balance throughout the range (excepting the frequency extremes where response falls off in comparision to tetrode) and smooth texture (mostly this makes itself apparent on transients such as vocal sibilants).

Things would be simpler if this was really all there was to tell, but that is not true. I left out what is arguably the most important advantage triode has over tetrode, but I did so for a reason. This advantage is in the way triode presents the harmonic structure, which is subtly but distinctly more ear-friendly than the emphasis given by tetrode mode. In this sense, think of flipping the mode switch as somewhat analogous to the idea of changing the nature of the light illuminating the subject of a photograph, its angle and diffusion. The cast lent to the harmonic series of overtones for any sound when in triode mode is perceptably more consonant, tetrode more dissonant. The result is that timbrally, triode sounds more natural.

The reason I chose to omit this distinction above - and it is a separate observation than the simpler question of overall tonal balance - is that I don't expect that this difference between the two modes will fundamentally change even if I install alternate power tube types. Rather, I suspect this phenomenon is inherent to the mode of tube operation. Don't get me wrong - the amp is still plenty listenable in tetrode mode, the slight harmonic hardness (compared to triode) not being obtrusive by itself, and less than other amps I've heard, tube or SS. I do think possibly a different tube type could alter this quality for better or worse - just not change the basic relationship between the two modes relative to one another, but again this is just what I suspect until I do the experiment.

If you haven't already guessed this by now, the total result of my auditions involving the mode switch have yielded paradoxical findings. I can summarize them this way: In all qualities 'physical', tetrode mode is the more convincing of the two. If, when you play a good recording, you try to imagine to what degree the sound reminds you of what it must have been like to be present at the recording session, triode will sound more like you're in an adjacent room to the music-making, listening through a fairly large aperture in the wall, while tetrode by comparision sounds more like you've walked through the door and entered the main room. BUT - and this is the paradox - the tonal and timbral quality of the music you hear in that main room (tetrode) actually sounds a bit more 'electronic' and less 'acoustic' than it did when you were on the other side of the wall (triode). Life is complicated...
Zaikesman and Trelja: I have both a pair of matched quads of EI KT90s, unused (add them to Joe's 4 and you've got your dozen), and a hoard of MO Valve/GEC Gold Lion KT88s, some slightly used. If you're really interested in trying them out to hear the differences (and, like Joe, I think that either of these tube types would be better than the 6550s, but that's personal preference), I'm sure we can work out some arrangement. Let me know, on or offline.
Nice to hear from you RCP. Just like in the majors, 3-way trades are pretty complicated to pull off...Besides, it doesn't sound like either of you guys has any questions about the Svet 6550C's you haven't already answered to your satisfaction? Since Trelja's using his KT-90's, I wouldn't presume disrupt his listening. Russ, if you are saying you're open to something like letting me give a 'workout' to 6 of your spares (for one monoblock - this would actually make for an easy comparision, as I can quickly swap one amp for the other and leave all the tubes in place, and believe it or not I actually prefer to make some critical auditioning tests in one-speaker mono) in exchange for the cost of shipping and a draft pick to be named later, I couldn't refuse. But if you are going to eventually sell these tubes and I don't wind up buying them from you, I wouldn't want to be the cause of you not being able to advertise them as brand new unused. Mail me if I'm in the ballpark...(apologies for corny running metaphor!)
Hey, I'm a fan of the best minor league team in the majors (the Mets, sigh) and am a follower of the team that stockpiles small forwards playing out of position (the Knicks, double sigh), so I'm used to that sort of thing. I'll e-mail you separately, but I can send you a 6-pack or two to try out.
Hey, I'm from need to gloat about losing teams (in both senses) to me :-)

Thanks for your email Russ, and when I've gotten somewhere with this experiment I'll be posting results here for all interested parties to check out (around a month from now most likely).
While we're commiserating, you guys know I'm from Philly.

The Sixers won the last title in this town - in 1983!!!

We haven't ever won a Super Bowl(makes you feel small when your neighbors are DC, Baltimore, and NYC), and the last NFL Championship was in 1960.

Ed Snider and Comcast run the Flyers, and Ed no longer is Ed, but "Mr. Snider". He returned to the team to snatch it away from his son, who Ed thought was doing a poor job, with the statement, "This is all BS, my goal has never been to get to the playoffs. That's chicken%$#^! My only definition of a sucessful season is the Stanley Cup!!!!" Nowadays, he berates the media at every turn, telling them, "What the hell do guys want? We make the playoffs every single year..."


Then they wonder why we're hooligans...

Honestly, Zaikesman, please don't worry about disrupting my system. While audiophilia is the antithesis of a team sport, cooperation and sharing definitely broadens my experience, enjoyment, and my low level of knowledge. Feel free to use the KT90s. My only requirement will be that I label them with a marker so I don't have to rebias the Jadis.
Zaikes, et al , let me alert you to a different KT88 than the usual suspects that I read about on the "Gon. This is the "Ruby" brand out of China.I was sceptical when a friend who works for the US distributor gave me a pair to try out in my ASL integrated but I was very pleased. They clearly beat out JJ/Tesla and EH in clarity without sounding etched. Plus he tells me they're cheap.check out John
Trelja, thanks for your kind offer. Russ is sending me six each of his spare KT-90's and MO KT-88's to try out in one monoblock against my 6550C's, and this should greatly help me get the way pointed right.

At least with the KT-90's, there's only one kind of the tube currently produced. As Jkaway notes, there are choices when it comes to 88's. I actually found that I preferred the Chinese Ruby 6SN7's over the Sovteks for the driver stage of my CJ MV-55 amp, but have never tried any Chinese power tubes, and generally haven't liked any Chinese 9-pin miniature dual triodes, so I was pleasantly surprised. I'll take a look at the website.
HEY GUY'S...(Zaik and RCP)....I'd be interested in the audition of the 88's vs the 6550's....
Please keep us posted!!
Jkaway, I've heard nothing but bad things about the Ruby KT-88's. Their EL-34's are junky, cheaply made tubes with thin glass, and the general consensus is that the Svetlana EL-34 blows it away. I find it very hard to believe the Ruby KT-88 beats the JJ-Tesla KT-88, which, along with the Svetlana KT-88, is the top contender for KT-88, in my opinion.
Kevziek, I should add that although the Ruby 6SN7's I tried sounded good in the application in which I used them, the envelope of one (the glass of which *was* fairly thin-seeming) came apart from the base when I attempted to remove the tube from its socket, and the Sovteks are presently back in there. I also agree with your statement about the Svet EL34, which is part of the reason why I did not hesitate to make their 6550C my uncontested choice at the time I originally retubed the VTL's, and who knows? - It (or their KT-88) could still wind up being the winner...
Kevziek, I appreciate your concern about reliability with the Rubys and can only say that there have been no problems so far (3 weeks). Actually, one of my JJ/Tesla KT88s shorted out recently,frying resistors in the amp so I was motivated to try something new.Eventually I will buy a replacement JJ to complete that pair and make another comparison.BTW the EH 88s were nice too but not on par with the Rubys.
Jkaway, sorry to hear about the JJ tube. Man, I'm getting tired of problems with tubes!
I also had JJ's (EL34's) go bad on me before I tried the Svet's, once taking a resistor. No such problems with Svet's or Sovteks, but the Svetlanas sounded best anyway.
Update: Russ did bring me (Literally brought them to me, 200 miles - Now how's that for service above and beyond the call of audiophile duty? [He had an 'errand' which took him by my neck of the woods] ) six each of his EI KT-90's and NOS Mullard KT-88's. Alas, the results of this noble effort and experiment are going to remain entirely inconclusive for now.

The KT-88's, whether because of their advanced age or something else (such as possibly differing somewhat from the current-day spec on the Svetlana KT-88's, which VTL assures me are a drop-in replacement that they've auditioned but rejected in favor of the Svet 6550C's I've been using), would not come close to biasing in correct range. They did this as a group, their individual biases all coming up in the same range as each other, so I doubt that any one of these tubes was 'bad', but whatever the cause, I couldn't do any auditioning with them, and I removed them from the amp after five minutes warm-up failed to budge them any closer to the desired 275-300mv from the 50-80mv they were registering with the bias trim-pots brought up gradually to maxed.

The KT-90 experience was even more aborted. The tubes I'm sure are fine, but new information has rendered this attempt a non-starter. When I first acquired these amps, I spoke to Luke Manley about a few things, one of which was tube selection, and in discussing the KT-90 option, he only addressed the sonic and reliability/availability differences VTL has found between these tubes and their chosen 6550C's. He discouraged me from going the KT-90 route, to me implying that they *could* be used, but what he apparently failed to mention at that time was that retrofitting KT-90's would entail resistor changes in the amp in order to work correctly. Oops. Having now found this out, and knowing in advance that this substitution wouldn't work out (meaning that at the very least, these tubes wouldn't bias correctly either) - and not wanting to risk damage to tubes not my own (or to my amps) - I haven't even tried putting them in the amp.

So, the one easy option available to me at this point would be trying a new set of Svet KT-88's, but I've got no clues as of yet (either experientially, or from VTL owners on this thread or in the archives) whether this would be worth me making the investment when I've still got a perfectly good set of 6550C's here. FWIW, Bea Lam at VTL says the KT-88's were passed over by them due for the most part to their softer bass response, although I think she liked their sound from the mids on up, but VTL thought the 6550C's were best overall.

There does remain the distinct possibility that the tonal balance I hear in tetrode with the 6550C's installed is actually largely accurate, and simply a reflection of my speakers' possible inherent tendencies. As usual, everything affects everything else, and unless I get a set of new KT-88's or eventually some different speakers (not an unlikely event in the course of time), my choice between running tetrode or triode for now will be based mostly on the demands and tonal balance of whatever recording I'm playing at the moment, with tetrode still getting the nod probably 90% of the time, despite the slight brightness/leaness.

That is, unless my next experiment renders this a moot point: I'm going to upgrade the amps' coupling capacitors soon, and from what I've been told, I could quite possibly expect some improvements in the very areas of tonal balance I've outlined here, along with even greater transparancy and purity, so maybe it's for the best that I don't worry about tube subs anyway until this mod is completed.

Many thanks again to Russ for doing everything he could to help me in this decision, and when he returns from his vacation soon, he and I will have to figure out how I'm going to get his tubes back to him...NYC road trip? :-)
Update: Bought and installed Electro-Harmonix KT-88's, after two of twelve Svetlana 6550C's failed at around 1 year and another had become noisy. I hadn't been thinking of going the Sovtek/EH route, until Kevin Deal at Upscale Audio recommended them as being the most durable new KT-88 while sounding just as good as the competition - that they're less expensive than Svet/SEC's to boot was only icing on the cake.

(Disclosure: I didn't wind up buying from Kevin this time, as his rather costly testing regimen hadn't seemed to correlate with longevity on the 6550C's. Supposedly, the internal Sovtek testing and matching regimen is much improved, so I bought from my usual source for new tubes at the best prices, Triode Electronics, but note in fairness that they don't claim to burn-in and pretest them the way Kevin does. So far, so good, but it's early yet.)

In my old amp that used EL-34's I hadn't prefered the sound of Sovteks vs. the Svet's, but they *were* durable (so were the Svet's, but not the JJ's). However, I am told that the EH EL-34 is indeed a slightly different tube from the Sovtek-labelled version, which is cheaper. Who really knows for sure, but I decided to give the brand another try, whichever name they put on this particular tube. The so-called KT-88EH's, while cheaper than SEC KT-88's, are more expensive than SEC 6550C's (and also of course than the so-called 6550EH's).

This comparison cannot be completely fair, for although my surviving 6550C's should still be in good shape at 1 year (and do bias well), obviously the new KT-88's should be expected to be a bit stronger. The fact that I'm switching brands as well as types also introduces another uncontrolled variable to the mix. One thing I can say for sure is that IMO, the fat-bottled KT-88's look more impressive just sitting there installed in the VTL's (though I suppose Freud might have something to say about that :-) Interestingly, Australian audio designer Patrick Turner, on his manufacturer's website, features a page devoted to his test results on the EH 6550, wherein he states that EH KT-88's tested at the same time displayed identical measured properties.

On to the sound: It was my sense that most users who had done the 6550/KT-88 comparison felt the KT-88 was a 'warmer-sounding' tube, but such was not my finding. Maybe these impressions were primarily formed in response to earlier 6550 versions, I don't know. In fact, Kevin Deal had said these two tubes wouldn't sound much different at all, and as it regards my amps anyway, he is mostly though not totally correct.

To reiterate what I was hoping for, I felt that the 6550C's might have been a touch bright through the presence range, and also a little lean from the lower mids down to the upper bass. But since first beginning this thread, I've come to the conclusion that perhaps my choice of input tubes was mostly to blame for this result. At the time I started this topic, my 12AT7 inputs were 60's Sylvania Gold Brands (also bought from Upscale), but they were more recently discovered to have become quite microphonic.

Bringing in a solid-state McCormack DNA-125 as a spare amp had highlighted for me how the sound of my reference VTL mono's had subtly deteriorated in ways that I had only really internalized subconsciously, and these flaws were found to be directly related to the input tubes. Those tubes were replaced, shortly before the output tube swap, with 60's Mullard ECC81's, that basically fixed some problems with tonal balance and imaging control, restoring to the entire presentation (even with the old 6550C's) a certain calmness and fullness that had slowly gone missing since the Sylvania's began losing it.

I've burned-in the KT-88's for about three days so far, and though they might still be changing, I feel I can report some results now. My first impression upon hearing the new tubes was that they had noticably more extended bass. Performing one-speaker-mono comparisons, where I'm switching one speaker cable between the two monoblocks, each fitted with a set of the competing tube types, has confirmed this finding head-to-head.

The bass region generally is where I've heard the biggest improvements to date. I am being careful to remember that my 6550C's have a year's use on them as I say this, but I never recall hearing bass out of them like I'm hearing now, even when new. The bass is not just more extended: it's also more evenly weighted, with a welcome equality of emphasis given to walking bass fiddle lines. This is both a matter of the improved deep bass extension and a concurrent reduction in mid-bass warmth that was probably spurious. The result is a bass presentation that is distinctly weightier, but no more preponderant overall. In addition, bass notes are more redolant of tonality with the KT-88's installed, while the 6550C's come across as having a bit of undifferentiated 'flat aspect' to their LF cast. All of these observed differences are fairly minor in degree taken by themselves, but together are important to better conveying the musical message.

Beyond the bass, the differences are primarily not ones of tonal or harmonic presentation. What I notice most is that the KT-88's give a slightly more distant spatial perspective, which I happen to prefer to the 6550C's more up-front presentation, and that they also tip the balance between direct and reverberent sound to more emphasize the qualities and dimensions of the recorded soundspace. In addition, individual images are better kept free of one another, with more space and less smearing between them. The only other qualities worth commenting on either way for each tube would be that the KT-88's seem a touch faster, while the 6550C's might be a smidge more extended on top (but I'm reluctant to draw final conclusions about that last observation, since I find that HF airiness is often the last aspect of a tube's sound to fully develop during the break-in process).

As of now, I am coming down in favor of the EH KT-88's (in my VTL 185's, through my Thiel CS2.2's, and for the time being with Audience Au24 serving as the reference speaker cabling) over the SEC 6550C's. Their (in my judgement) superior bass fidelity, coupled with a more separated and laid-back spatial presentation, better press my personal sonic hot buttons.

But I have not found this tube to sound any warmer than the 6550C's - to the contrary, if anything they are a touch cooler, a little more analytical and removed-sounding. You could even say that the KT-88EH's sound a little more like solid-state, the SEC 6550C's a little 'tubier', but again any such differences are very minor everywhere above the mid-bass. Anyway, my experiments to date with input tubes convince me that this is the tube position where one should focus on the tonal and harmonic balances, and I may yet try something a touch warmer here, because the Mullards sound quite hi-fi but not warm themselves.

Besides the generally advantageous sonic qualities I've described, I'm also hoping that the KT-88EH's will prove more durable compared to my so-so experience with the 6550C's, but that only time will tell.

P.S. - My thanks again to Rcprince for making the effort in trying to assist me with auditioning some of his KT-88's before having to plunk down for 'em myself - despite the unforseen complications, I think it's all worked out in the end Russ! (I'll be sending you a tracking number tomorrow :-)
Zaiks....Thanks for the update!!!!!
I am sure the KT-88EH's are fully run-in now, and there are some new findings to report. Specifically, my initial impression that there wasn't much to comment on regarding differences above the bass range needs amending.

Further auditioning has made it clear what some differences are. The 6550C's sound distinctly 'creamier' on for instance vocals, while the KT-88EH's transmit more detail and nuance and generally sound less 'tubey'. The apparently higher fidelity of the KT-88EH is maintained up the frequency spectrum, where cymbals display a bit more metallic character and harmonic detail, while the 6550C's present a less distinguishable hiss that is more akin to white noise.

Some may find the KT-88EH sound less to their liking than I, since it is not overtly warm or liquid in the way that many audiophiles prefer tubes to be. But, the combination of some of the factors I have heard - better bass, more detailed articulation, more spatial specificity - have me becoming cautious with my conclusions, thinking that maybe the year's worth of prior use on the 6550C's is affecting the comparitive results more than I have supposed. I think in order to make a fair playing field that I will have to use the KT-88EH's for a year while the 6550C's sit in their boxes, and then return to the comparisons to see if my impressions will still hold to the same degree then.

What I can say is that the combo of these new power tubes, the new NOS input tubes, and the new input tube dampers - combined with me finally getting the same resistor upgrade done to the remaining speaker that was done on its pair-mate last spring (in which the underspec'ed resistor had failed completely) - has given me the best sound I've heard from this amp/speaker combo so far. I also think the Au24 SC's - whose attributes vs. the Satori are actually not unlike those of the KT-88EH vs. the 6550C - are here to stay, along with a switch from HT Pro-Silway II's to Van Den Hul The Second IC's leading into the amps. The sound is now faster and more open (and has more depth) while being less warm, but it is also less bright, and much more solid throughout the bass (that being due mostly to the KT-88EH's and the VDH IC). As a bonus, I have inklings, yet to be fully tested, that the KT-88EH's are more competent working in triode mode than were the 6550C's, and the sound of the two modes is actually closer together now in many ways, none of which I think are penalties.
Zaikesman, it's commendable of you to keep us up to speed. Thank you so much.

Yes, Russ sure is a prince. Heck of a guy!

I'm happy to hear your impressions on the tubes. I have never given KT88EH tubes a listen, but in my experience, the JJ(Tela/Teslovak) KT88 are easily much sweeter than any 6500 I have been around.

It's so wonderful to hear that you are getting the best sound you have ever had!

Yes Zaik, I second Trelja's comments about your commitment to keep us updated on your testing and comparisons. I must say, I now am considering some of these KT 88's in my ASL Hurricanes. Anyone out there make a change to their 88's in the 'canes?
Rwd: The EH's are the only KT-88's I've tried or heard. Which ones are in your Hurricanes?

Trelja: In my amps, in my system, I can't really say that the KT-88EH's are "sweeter" in any way than the Svet 6550C's, mostly just more 'complete'-sounding: more full-band, more even-handed and neutral in frequency response, faster and cleaner, more spatially resolving, and more transparent/detailed. But as I said, I'll have to revisit this comparo after the new tubes get a year's worth of use on them to make it an entirely fair fight, so although I'm convinced the sound I'm getting now is 'better' than what I achieved at any point in the past with the 6550C's, I'm regarding my specific characterizations as provisional.
In my Hurricanes I have the stock KT 88's.....Value Art
Are Valve Art Chinese? I haven't seen them being sold in the usual places I go for tubes, and haven't gotten any input on their sound.
I changed the Valve Art KT88s in my 'Canes to the EH KT88s. A definate improvement in detail and frequency extremes. I haven't heard the 6550s in the amps, but I feel that I'm not missing anything at all with the EH KT88s. They have all the bass I need, the highs are transparent and airy, and the midband is very good and natural sounding, certainly not overly "tubey" or syrupy at all.

The biggest bang for the buck, however, would be to change the input tubes and drivers, which will alter the sonics more drastically then the output tubes.
Hi Mark...yes I also change the 6sn7's ....BIG difference!

Zaik...yes they are Chinese tubes.
1markr: Sounds like you and I have a pretty similar take on the sound of the KT-88EH. Since the Hurricane's haven't been around all that long, I'll assume your original Valve Arts weren't worn out when you made the switch, meaning the EH's could well be a superior version of this tube. Anyone compared the Svet/SED or JJ/Tesla KT-88's directly to the EH's?
I got my Hurricanes last summer/fall. My original Valve Art KT88s had maybe 200 to 300 hours on them, so much more time is left on them. A couple of the VA blew, and I replaced them with the Electro Harmonix, and before you know it, both amps became fully populated with the EH. Mind you, I have not blown a single EH KT88 since they have been installed on the 'Canes.

How long are these power tubes supposed to last? I do most of my listening in the triode mode, but I'm not sure if this affects the life or not?

I'm guessing the tubes ought to be good for up to 3,000 hours or so, but it depends on the circuit they're used in, and ultimately it's a matter of degree anyway, since tubes technically begin wearing out as soon as they're finished breaking in. As I think I mentioned above, Kevin Deal at Upscale Audio told me he thought the EH version of this tube was the most durable going today. Time will tell...