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.


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Wow, very cool that you put the time here to share this. I have seriously thought of doing this too, but the added height of the Kt66 tubes means I either need to leave the cover off the PS (not good due to 2 cats roaming around and occasionaly visiting children), or to modify the cover to allow for the pair of tubes to peak through. I guess your coverage on this yet again means I should give it a try. Thanks Chris.
I am quite surprised that Jim White never tried replacing the El-34's in the power supply with the Genelex KT-66. If I were a designer and manufacture of a particular piece of equipment I would have tried every combination possible to obtain the highest possible performance from that unit. With all of the time and effort taken to produce the Aesthetix Callisto and Io and to market them it is remarkable no effort has been made to even try to improve on the stock unit with improved tubes and connectors. Many of these NOS tubes while expensive are still available yet apparently Jim only uses stock Russian tubes.

The Aesthetix Io and Callisto are fine products but I still wonder why more designers have not tried or don’t care about upgrading tubes or improving the female rca connectors for better performance. The fact "he said has never tried it" when other customers have with very positive results is quite baffling to me.

42659: I fully understand your point here and this has much to do with why I have replaced all the stock Sovtek tubes with various Mullard, Telefunken, GE, etc, in my Io and Callisto. However, I also understand that a manufacturer must go with new parts that are readily available off the shelf. And for mass production, this is simply not possible with all the older tubes that makes these units sing their magic.

If there was to be a run of 10, 20, 50, or so of such units, these could have been delivered with hand picked premium tubes with the owner being made aware that such tubes, and therefore that same performance, might not be available when tube replacement is necessary. Such tweaking comes down to the responsibility of the owner, and for those of us willing to do this, the opportunity to benefit more with these units is available for the taking.

One final note: I can say the exact same thing about the CAT Ultimate preamp. A replacement of the stock Sovtek tubes to the Mullard takes this unit to a whole new world. For those that claim that the units were voiced with the Sovteks....all I can say is, whatever.

I have done the KT66 tube substitute for years, I posted this five years ago at Audiogon.

I still have the special tool that will punch the power supply chassis so the KT66 will clear.
42659: Just to ad a bit to what Jafox is saying; I was not kidding (alright a little) about the Genelex KT-66's being part of my portfolio....... I have seen these tubes selling for $600-$700 per pair in some places. If you have a dual power supply like Albert (John I'm not sure about your setup) that would ad up VERY quickly. It would be impossible for a manufacturer to do something like this........ Like Jafox say's; you have to be able to stock replacement parts (in this case spare tubes).

Albert, I wonder if you could talk a little more about the sonic differences in the 6922 valves you have tried?. The only rolling I have done in this regard is with the Mullard gold pins.

Someday I want to purchase an IO. Do the same tube replacements have a similar effect in that unit(incrementally speaking)?
Yes your correct Cmo that the KT66's can be expensive and could not be done for production units but what surprised me is that Jim White never experimented with his own unit to see how these tubes would sound with his own system even after some people including Mr. Albert porter reported back with very positive results from changing the tubes. If I was a designer I would like to satisfy my own curiosity and know for my self how these tubes improved the sound of my preamp. Other improvements like upgrading the female rca’s jacks to pure copper Vampire’s (the stock cardas sound very poor in comparison) and changing the male IEC in the power supply to the much better sounding Furutech are not that expensive and made a very nice improvement on my Jadis JPS 2 preamp.

The Mullard (dimple plate) gold pin is what I run in my Aesthetix Callisto. In the Aesthetix Io, the 6922 of choice is the old diamond bottom Telefunken 6DJ8.

Oddly enough, the Telefunken does not sound as good in the Callisto nor does the Mullard sound as good in the Io. Although the same 6922 tube is supplied from Aesthetix, they perform different jobs in the two units. In the Callisto the 6922 is paired with either 5692's or 12AX7's depending on input or output side.

In the Io it's in the output and not directly paired with another tube (as near as I can tell from the circuit). So as usual, the answer is not always tube "X" is best, depends on where it goes.

I don't know if the link I provided in my earlier post opens where you can read all the comments. I had problems clicking on other posts from within my link and got a server error. If that did not make it, I can copy and post it here. I discussed at length the differences between Siemens, Amperex, Mullard and others.

AS always thanks for your input on all things audio. I was unable to open many of the comments in the link you provided either. If you have time I am intersted in your findings on the various tube swaps you mention in your earlier post.

Finally I found a little loose change so I could not resist trying the KT66 tubes after Chris’s high praise. I got a used pair of Genalex and an unused pair of Marconi KT66. Currently I run with only one P.S. for the Callisto as well as one for the Io. Four large chassis here are tough enough to find available space….I have no idea where I could store two more of these beasts. And so I have held out for Michael Elliot’s preamp to determine if I go that way or an all-out 6-chassis Aesthetix setup.

After trying out several EL34 tubes last year in the Callisto and Io, two clear winners stood out. My favorite was the UK Mullard. As I reported on another thread, this made a huge improvement which set the stage to replace all the 12ax7 and 6922/6DJ8 tubes in the audio chassis. The fatigue and edge of the stock Sovtek EL34 was gone. The other tube, which is much less expensive and readily available is the JJ Tesla EL34. I was reluctant to try the JJ Tesla as I had major reliability problems with JJ Tesla KT88 tubes a few years ago with an ARC VT130 amp. But the JJ Tesla EL34 is really mighty fine in terms of keeping up with the UK Mullard’s tonality and dimensionality strengths. When I did this KT66 comparison test last night, I had forgotten how mighty good the Tesla EL34 can be.

The new reissue Mullard EL34’s have a wonderful coherent tonality. But they are absolutely flat in dimensionality. Gone is the incredible harmonic richness, ambience and front-to-back placement as portrayed so well with the UK Mullard and Tesla. I also tried the Svetlana and Electro-Harmonix EL34 but these too were rather lifeless much like the reissue Mullard. In a budget pinch, the JJ Tesla EL34 would be the clear choice but that last wee bit of portrayal of space and openness with the UK Mullard brings on more of the goose bumps factor.

And now the KT66 experience. I started with a pair of UK Mullard in the Callisto and the Tesla EL34 in the Io. First off was to focus on the Callisto. Two LPs were used – Alan Parsons Gaucho with singer Lenny Zakatek and a Carly Simon compilation LP. Both of these singers have a wonderful range with great body to the tone of their voice. After a few listens to Gaucho, I switched to the Genalex KT66 in the Callisto. The difference was immediate. Whereas the UK Mullard portrayed the music as “right there”, the Genalex portrayed the musicians far far distant. It was like I had to really listen hard to bring them into the room and enjoy the music. Tonality was fine but that immediacy was simply gone. I did not hear any detail or extension at the frequency extremes with the Genalex over the UK Mullard. About ½ hour later, I returned to the UK Mullard EL34 and the magic was back. My thought was that perhaps the Genalex or even the KT66 tube was not a good match here. I then dropped in the Marconi KT66. Wow, this too was a huge change. And it was quite the opposite to the Genalex. The Marconi was quite vivid with a huge projection into the room. A few tracks later and it had relaxed a bit more to my liking. And there was indeed more low frequency presence with the Marconi but to me it was more like one-note thumping rather than a bass range that integrated well with the rest of the range. A return to the UK Mullard brought a return to tonal coherency…..and a more natural top end. And with the latter came more spatial cues and ambience. In the final analysis with the Callisto, the Genalex was way too distant and the Marconi which initially was vivid and rich became too fatiguing and lacked the refinements of the UK Mullard. And the Marconi was quite a ways behind the Tesla EL34 as well.

I left a pair of Tesla EL34 in the Callisto and started with UK Mullard in the Io. The focus now was to change tubes in the Io. To make a long story short, the Genalex and Marconi exhibited exactly the same sonic signatures with the Io PS as they had with the Callisto PS. The same tubes can be polar opposites between the Callisto and Io audio chassis but here in their power supply chassis, the result was identical. The Genelax was way too mellow and distant and the Marconi was again too forward on top which got in the way of the music’s dimensionality …. and the bass with the Marconi was also boomy and just not natural. A return to either the UK Mullard or Tesla EL34 in either or both power supply chassis brought on a degree of musicality that the KT66 tubes simply did not deliver for me. Oh well…..I was so excited going into this trial but in the end, the lowly EL34 tubes survived being shelved.

On the comment by Albert on the small signal tubes, wow, my observations match so well. For the Io, I just love the Tele 12ax7 throughout. But I also find the Ei 12ax7 tubes work well on the first input stage as they are super quiet. But with the Callisto, the Tele 12ax7 is so boring and dull. I need Mullard 12ax7’s here to bring on the dynamics and life on the top. And then the issue of 6922/6DJ8. I have been running with gold pin Mullard 6922 on the Callisto and Io but just last week I finally got a chance to try the Tele 6DJ8. A quad of these in the Callisto was too much of a good thing but I really liked a pair of these and a pair of the Mullards for a balance between dynamics and tonal coherency to the trebles with a nice bloom in the mids. But the huge surprise was the pair of Tele 6DJ8 in the Io. WOW! This was as dramatic over the Mullard 6922 here as the Mullard 12ax7 was over the Tele 12ax7 in the Callisto. It is amazing how a few select pairs of tubes out of 80+ in the system can make such a huge difference vs. the others. You really need to try out as many as you can and all over to make these little discoveries.

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.

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.

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.