Life continues to get more interesting with each passing day. Here are some of the things I'll be sharing with you in this installment of recidivist audio incorporated... VTL recently sent me a set of closely matched KT-90's for evaluation - stay tuned for the review. I recently acquired two pairs of Merritt deJong audio interconnect cables - and will they give the XLO interconnect a run for the money at a much lower overall cost (wallet relief) - read on to find out... Comments on the Museatex Melior Transport and D/A converter.... Tips on those crudy old germanium diodes you find in old Scott and McIntosh tuners, selenium rectifiers and why they aren't your friend, and some more thoughts on neat vintage stuff.
I'll start with the VTL KT-90's, they are performing flawlessly so far and their performance is a significant enhancement over the Tungsol 6550's I was previously using. The Citation II that I use on a day to day basis operates solely in triode mode and features differential driver circuitry featuring a 12AX7 used as a cathode coupled phase inverter and a 12BH7 as the differential driver stage as well as isolated regulated supplies for the driver stages. Unregulated DC powers the filaments of the driver stages, and several thousand uFd are utilized in the voltage doubler and filtering to the output stage. B+ is augmented through the use of a series transformer to boost the B+ to approximately +560V. All the low current supplies to the driver circuitry are regulated. It is extremely fast, transparent, open and detailed. In short a pretty good device for evaluating output tubes for ruggedness and sonic quality.....
I also currently have a pair of Kennedy Audio MKIII triode amplifiers on hand as well. Topologically this amplifier is very similar to the Citation except that the power supply utilizes a 5AR4/GZ34 instead of solid state rectification, and obviously the Dynaco transformer is somewhat different too. Maximum output power is somewhat reduced at 25 watts rms, but they do sound remarkably similar in many respects. These amplifiers image extremely well, perhaps in some respects even better than the Citation II due to their fully mono construction.
The most important aspect of a review is to determine whether the item being reviewed fulfills its purpose adequately and deserves a recommendation. The KT-90 is a superior performing output tube in appropriate applications. This tube is a suitable replacement for the 6550 in the Citation II, and performs extremely well in my Dyna MKIII triode amplifier as well. This tube provides enhanced performance across the spectrum when triode connected, but the largest notable difference is what it does for dynamics - this tube provides PUNCH when called for and subtle control all at the same time. The bottom end is solid with none of the flab evident with the old Mullard KT-88, and yet at the same time the highs seem to extend into the statosphere with an ease that is not matched by very many modern tubes. Depth and soundstaging are all enhanced as is the pervasive sense of transient speed and agility. Timbre seems dead on, and I have never really heard the subtle layering and harmonizing produced by good Jazz or Classical horn sections more cleanly or more beguilingly reproduced. (Read: lots of detail, NO intermodulation induced smearing, clean, natural tonal balance, etc.) Subjectively both amplifiers sound cleaner and more powerful, more capable of handling the dynamic contrasts of music, the subtle detail of my best LP's and CD's, and the sometimes potentially analytical nature of my MG1.4's.. Does this sound like a rave review? It should!
Electrically the KT-90's were matched to extremely tight tolerances (VTL is to be commended on this one) and the measured transconductance was typically 9000 umhos which theoretically is significantly lower than the 6550. (Note that I measured only 7000 on a new GE 6550A.) Referring to the Sylvania tube chart it would seem that the KT-90 probably is NOT a universal replacement for the 6550 or KT-88 in all applications due to significantly higher plate resistance (Rp). The plate resistance and therefore probably the optimum load resistance is significantly higher with the KT-90 operating in pentode mode, but triode mode is another matter altogether, as it appears that the KT-90 matches extremely closely the characteristics of triode connected 6550's. If at all possible I would advise that you try before you buy. See the table:
The maximum values cited are design center values, and should not be exceeded in normal operation. All other parameters are measured at a Vp of 250 V and are for comparison only.
The next topic I would like to cover is a new audio interconnect cable that is so good I can hardly believe it.. Not only does it provide a level of performance that is close to state of the art - it does it at a low, low price. No Longer Available 2/98 I am talking about the new cable from Merritt de Jong Audio, 5490 East Oak St., Evansville, Ind. 47715, Tel: (812) 477- 4055. This interconnect blew away everything (including me) in my system including the XLO interconnect and only costs $70.00 per meter pair terminated, and each additional meter costs only $10.00 extra. The MdJ cable was initially compared with the Radio-Shack Mega cable I was using, but there was no contest in terms of which cable was better, and so the Mega Cable was discarded as a reference, and XLO cable was used instead. The XLO cable used as the reference sounded somewhat congested and glassy in the mid range, and phasey, hissy and diffuse in the highs, but substitution of the MdJ cable netted an immediate improvement in both instrumental textures and detail in my system, and in the system of a friend. The highs seem much more coherent and cleaner, instrumental textures more vivid, transients snappier, the mids more lucid and open, although the bass response subjectively seems somewhat attenuated compared to the XLO, and less extended. The overall low frequency balance is still excellent and perhaps more natural sounding than the XLO. This interconnect does not have any particular emphasis anywhere in the audio spectrum, and is extremely neutral overall. This is my new reference cable. An excellent speaker cable is also available, but this like any speaker or interconnect cable should be carefully auditioned prior to purchase to determine its compatibility with your system. Please note that the XLO and MdJ cables were both broken in with a Duotech Cable Enhancer for about 40 hours each. (Not by me however.)
I have not had the opportunity to seriously audition the Museatex Melior transport and D/A converter for an extended period, but the music they made in my system convince me that they are quite excellent, and the improvement in CD sound quality in my system was at least as pronounced as the change in my interconnects. They handled the most difficult passages in my Mercury Respighi/ Pines & Fountains of Rome with aplomb. Strings had texture and detail I have never heard from my heavily modified Sony CD-790, and the strident string sound I took for granted on this CD is definitely an indication of stress in the Sony and not a short-coming in the recording itself. There was a palpable soundstage, presence, and a warmth entirely lacking in the presentation of the Sony. I hope to have the opportunity to investigate further, and would seriously recommend that anyone considering the purchase of a converter/transport combo in the under $3000.00 range give them a listen.
Onwards, ever onwards.... If you have ever serviced a tuner you may be wondering whether those crummy germanium diodes 1NXX can be replaced with something else. I have found that 1N4148's work just fine in the majority of applications, but I do recommend matching thresholds to better than 5% for discriminator, ratio detector or stereo demodulator applications. If possible you may want to match their junction capacitance when reverse biased. Hewlett-Packard Schottky diodes also work well in these applications.
About those selenium rectifiers - many of them by this point just don't work. Replace them with appropriately rated silicon rectifiers in most amplifiers and tuners and you'll be surprised how much trouble this will save. Like blown output tubes due to inadequate bias in Dyna, HK, Marantz, Eico and other amplifiers, tuners that don't work like the McIntosh MR-71, pre-amplifiers with no gain like the Scott LK-130 and PAS2/3...
For those of you who own any Scott or Fisher gear look out for American Radionics Ceracaps as they are quite unreliable - the leads tend to separate from the foil in these things (they are film caps) and they become intermittent usually or totally open (rare unfortunately) replacement with CDE WMF mylar caps will restore performance and are sonically as good or better than the originals. Smaller Ceracaps can be replaced with Mallory SX or SXM series polystyrenes.
And now here's a list of some of my favorite things, excerpted from my little audio handbook:
©1998 By Kevin R. Kennedy