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Review: Aesthetix Callisto Signature and the KT-66 Tube preamp

Category: Preamps

This is not necessarily a review of the Callisto Signature itself (that has already been done), but rather relates to some new tube rolling options I have recently discovered. More specifically, the KT-66 tube as a replacement to the EL-34 that typically is used in the unit.

To start I listen to all kinds of music, from Rock and Blues, to jazz, County (early), singer song writer, Bluegrass, some Classical, and Rap/Hip Hop. I really enjoy a variety of recordings too, from early field recordings of Muddy Waters, to purist audiophile stuff like Analogue Productions (the new 45 rpm Jazz series is great, BTW). I am mainly an analog guy and will go out of my way to puchase a record if I can (over the CD version)...... It's fair to say that I have listened to all of these kinds of recordings during my time with this preamp.

I place an emphisis on musicality, resolution (not necessarily detail), and most of all tonality. The other audiophile stuff, like soundstaging and PRAT are certainly factors but come in on down the list. You can check out the link to "My system"......... I'm into tubes and have just found my way to SET amps and horns after a love affair with Electrostats and big tubes (Martin Logan Prodigy's and ARC VT200 for anyone wondering).

I bought the Aesthetix Callisto Signature preamp to replace my ARC LS25II some months back. It has been a major sonic upgrade for me. A couple of months ago my (obsessive) curiosity got the best of me and I figured I would try some tube rolling in the unit. Background: I am a professional musician/guitar player and own (have owned) quite a few vintage amps.......... Well, I had some NOS tubes around from an old VOX AC30 and from a '59 Fender Twin so I figured what the heck. First I put in a pair of Amperex EL-34 Bugle Boy's (probably mid '60's vintage)... the sound took a huge leap forward in liquidity and smoothness. I had more resolution, a lot better bottom end and the high's were much "prettier". In essence everything got a little bit better.........

I guess I couldn't leave well enough alone. I have a quartet of NOS Genelex KT-66's from the Fender that I have been saving to put in my portfolio for when the kids need that education. They are not a direct replacement (and I'm not a tube expert) but I have heard of people swapping them in their guitar amps, etc. I emailed Jim White and got his opinion...... He said he had never tried it but has had some customers who swear by the substitution. At this point I figured I was on to something interesting.....

When I put the KT-66's in everything went up another notch...... Maybe three or four notches actually. Everything I heard with the Bugle Boy's was multiplied. The biggest differences though were in the bottom end extension. Thunderous bass, tight and big. It sounded like I had installed a sub woofer. The highs are very sweet and extended........ And the mids are just creamy and dreamy. These tubes are a bit quieter too. I don't know if this is inherent or just my samples.

After all of this, I took Albert Porter and Jfox's advice on further tube substitutions..... I am now using Mullard gold pin 6922's, RCA 5692 (6SN7's), Tele 12AX7's.... These substitutions have made a great sonic impact as well, taking the amp to yet another level of performance.

This thread is about the KT-66 though..... I recently put the Bugle Boy's back in for a reference after the other tube swaps. No comparison, the KT-66's are a huge improvement for me. All as mentioned above but maybe even magnified with the NOS tubes.......

So, for you fellow Callisto owners, if you want to try an interesting experiment you may like what you hear.

Caveats: Per Jim White: "I have friends that have successfully substituted KT66s. I don't generally void the warranty unless the substituted tube causes the failure, which I can nearly always determine by the failure. We have had few cases like that in over 10 years of production"...... I have been running them for about three months with no problems. One more thing, becouse they are a lot bigger than the EL-34's the cover will not fit on the pre either.

Anyway, I hope this might provide us with some discussion and other interesting findings in reference to this great preamp.

Chris

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cmo
cmo OP
471 posts
 
Wow John, sure sorry to have steered you wrong on this one........ I wonder what the deal is? The sound you are describing is certainly not what I am hearing in my system. The Kt-66 does give a "slightly" more laid back presentation, but I would just describe it as a deeper soundstage. Are the Genelex' you are using grey glass? ..... Anyway, if you ever want to sell those babies I might be interested.

Maybe Albert will chime in on this one again to shed some light.

Chris
jafox
1,447 posts
 
Hi Chris, no no, you did not steer me wrong at all - on the contrary. Only through posts like yours here are the rest of us made aware of what else is out there to try. I greatly appreciate the time you took to share here.

As for the KT66 not working out for me, I suspect it has to do with me liking the presentation brought on by the UK Mullard. Like the Tele 6DJ8 in the Io and the Mullard 12ax7 in the Callisto, some things just lock in for me.....and the rest of my system. And keep in mind I have gone through a lot of effort the last year to re-cable my system and the EL34's were during this time. Had my system had either of the KT66 tubes, the resultant cables could have been very different.

Had I gone from the russian EL34's to the Marconi at the start of owing these Aesthetix products, the Marconi would have been a clear step in the right direction. But the Genalex would for surely have been too mellow for me. My system has been more in the warm and mellow zone vs. a highly resolving and upfront presentation, and so the Marconi would be the more suitable tube here. But one thing for sure - these two KT66 models were not at all similar; the EL34s are very much in the middle in terms of tonality and stage placement.

As for selling these, I will check with Michael Elliot to find out if these would work as a viable tube in his upcoming preamp power supply. If I want to go a little more mellow or have a greater projection into the room, I may have the perfect tube to take me in one direction or the other. Or the stock tube may also lock in. So I will let you know and then pass them onto you if your interested. And yes, the Genalex is the grey glass and the Marconi is clear. The Marconi powered up is a beautiful tube to see.

John
albertporter's avatar
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albertporter
4,949 posts
 
I used the GEC KT66 and loved what they did for textures, warmth and midrange magic. They sound very slightly compressed but incredibly emotional, especially on female vocals and jazz.

Bandwidth wise they are not as accurate as genuine Mullard Xf2 EL34, at least in my system.

I actually run Genelex KT77 in the Io phono and Mullard Xf2 EL34 in the Callisto preamp. This wound up the best combo of all for balanced sound. I always wanted to try KT77's in all four power supplies but locating that many is difficult, not to mention the cost.

As for KT66 not clearing, that is true. If you decide to stay with that tube as I did some years ago (before switching to current configuration) I still have the proper size metal punch to make a hole in the power supply covers to clear the tube.

Once punched the KT66 Genelex barely protrudes above the cage. I frequently wiped it with a dust cloth when cleaning the top of the power supplies. Later after experimenting with the Genelex KT77 and Xf2 EL34 Mullard combo and preferring the sound, I never punched the chassis on the Aesthetix Signature versions when I upgraded.

A lot of tube choices are tied to personal taste, associated tubes, power cords, isolation devices, the other components in the system and even the room acoustics. It's great to exchange ideas so we have a direction to go and find what works for each one of us.
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