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 :-)
(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 :-)