time to discuss kt-88's

I have a suggestion of another avenue. I have tried Sovtek, Svetlana, JJ, and other KT-88's in the past in my Sonic Frontiers SFS-80. I mention this due to it using a pair of output tubes per channel as well. I was constantly tube rolling and was talking with Kevin @ Upscale Audio and he suggested I try Ei KT-90 type III tubes. He also warned the drawback of taking 400 hours to burn in and fully open up. Well, after tracking hours and about 385 hours into play time on the new tubes I was ready to give up. The mids were a bit analytical and not as open, the bass prowess was much better than any KT-88 I had tried, and the top end was clean and extended. Yet without the mids being open and relaxed, why have a tube amp? Uncannily a few days later while in the office, I got off the phone and noticed the piano (George Winston) sounded THERE... and I was down the hall a few rooms. I kid you not - 412 hours logged and there was a transformation! The sound was what I was searching for over many, many tubes. Overall, the bass was the most controlled and defined, the mids were open, airy, hamonically correct, with clean & extended highs. They were the most neutral of the lot and the least colored. The Ei KT-90 type III's also produced the best macro and micro dynamics.

I will mention the midrange was best with NOS GEC (Genalex) KT-88's from 1966... however quite expensive and a bit colored, with sloppier bass and highs a bit rolled off.

Currently I ordered my Quicksilver V4's with the Ei KT-90's from my experiences in the past. I haven't questioned my power tube choice once.

Some find the KT-90 to be more "analytical" sounding (and THEY ARE for the first 400 hours or so) but after that it seems to be extremely neutral. I tend to prefer to use NOS Mullards in the preamp, or a tube that is slightly warmer to "tune" the overall sound to your preference. That way you get the best control of bass, dynamics and a neutral power tube, and can tube roll relatively inexpensive on the preamp tubes.

You may want to make a call to Upscale Audio and get their advice on KT-88 / KT-90 comparison.

Good luck on your search!

Thanks, guys. I'm still surprised there isn't more folks here who want to discuss kt-88's.
The first time I tried the silver 90's was a short trial, and they had svetlanas. I remember the sound as being REALLY clear and smooth in the midrange, but soft in the bass and lacking the deepest bass completely, and vary soft on top. While I was really unsatisfied with these shortcomings, the mid was so beautiful that I proclaimed "these amps could actually get a guy laid".
The silver 90's I have now came with kt-90's, and I did not like them at all. They also came with cv378 rectifiers. I soon replaced them with chinese kt-88's and then was pleased, as the highs came back I was lacking with the kt90's, and the sound opened up the way I knew it should. I then replaced the rectifiers to the 5ar4's it called for and the sound became tighter and punchier. Did not seem to have the mid range clarity I remembered with the first pair though.
I got on the web with upscale audio, (that is where the tubes were sourced), and to my surprise they had this 'cv378' rectifier where it explaines you may "lose highs and lows", along with some plate voltage warnings. I put the kt90's back in (with the 5ar4's) and no longer do they have the severe lack of highs and definition they had before. In fact, they seem to have very good definition. They are very detailed, have good presence and 'punch', and great bass. they are, however, real flat in dimension and not very open sounding.
I thought that perhaps they were starting to burn in some when I read audiofankj's post. They are starting to sound a lot better, some 30 hrs later. I think I maybe played them for 150 to 300 hours when I got the amps, and I don't know how long before I got them, so I will continue to let them cook. I might be living on the edge, though, as it seems the reason for the cv378 rectifiers was to lower the plate voltages so the amps could handle them. The amps call for 100ma with kt-88, but with kt-90's, the plates on one of the tubes in each pair glow just a LITTLE in the crease, so I have them at 90ma to minimize this.

Hey Basement - you can also verify with Kevin @ Upscale Audio, but I do think I was told 400+ hours on the KT-90's... and that is not the easiest amount of time to log on tube amps! However, when you do reach that point over 400 hours, you will know it. It was not a subtle difference. The somewhat "closed in soundstage" will also tend to open up as well. It may not be the best in the regards, but much more open as compared to when they are new. The mids and highs remain neutral, however seemingly silky smooth & clean. The bass will seem to have more "punch" and refinement as well when you cross the break in thresh hold.

Please keep us posted. Good luck!

Tubes taking 400 hrs. to burn in? That is completely ridiculous. Electrons traveling in a vacuum don't "burn in." If you argue that cathode & anode burn in, that is going to happen within a few minutes from new. You're probably just getting use to the sound of the tubes, or they are changing in sound because they have already aged in 400 hours.

As far as comparing KT-88s, this will differ from amp to amp. You need to get recommendations from Quicksilver owners, or from Quicksilver themselves. In the ARC VT-100 Mk II, I found the Svetlana to have a beguiling sound with some pleasant 'tube fog' in the midrange. Decent, but not outstanding bass, silky treble that does not have the ultimate in detail however. The EH KT-88 is a very good tube, as is the JJ KT-88. I would try those in the Quicksilvers, as Quicksilvers have had a reputation for a very midrangy, bloomy sound that could use some bass tightening and high end extension.

Yes, 400 hrs for a tube to burn in IS rediculas, but I have experienced rediculasly long burn in periods in other areas.
I did call upscale to ask about this, and they more or less confirmed this, saying about 300 hrs, and that they have an "unusally" long burn in.
My assesment of the svetlanas is not nessesarily different, but in the quicks, what I heard was definitely a very 'clear' midrange, in fact, was able to pick up a lot of the harsh, buzziness that a real trumpet or saxaphone sounds like. Synergy is sometimes matching strenghts and weaknesses, and sometimes it is just a unique situation. It is worthwhile to note or discuss what tubes work well in what amp, and of coarse while assesing a tube the amp it is in must be considered, but there are still differences in tubes and brands that make them stand out or just be so-so. It is interesting that the svetlana, that seems to have such a glorious midrange and soft and soggy highs and lows would not balance out in an arc, but this could be telling, too.
The kt-90's ARE changing to me, which I will comment on later, and I should also note that in my experience, the quicksilvers to me are not amps that have long burn in periods with different tubes or long warm-up times.
I will continue to burn these tubes in and report back soon.