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.