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.


Associated gear
Click to view my Virtual System
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.