Kit 1 was my second Audio Note Kits’ product that I built (first being the DAC 2.1). It was a bit more complicated but I was rewarded with the beautiful life like sound of an excellent implementation of 300B direct heated triode tubes operating in the single-ended mode. The Kit 1 that I purchased was a Signature version. It came with Audio Note UK (AN UK)Tantalum resistors, some Black Gate electrolytic capacitors for low voltage applications and AN UK copper film capacitors for signal coupling. I pretty much left most of it as stock except the following.
I added more Black Gates to the extent possible even in some of the high voltage areas where appropriately rated capacitors were available.
I rebuilt the rectifier bridge with CREE silicon carbide Schottky diodes. The degree of sound improvement from this change can vary depending on the equipment but it is always a worthwhile upgrade for me. The first time I made this upgrade was to the LiTe DAC-AH. I was very surprised how much the sound improved. It opened up the sound that reviled details and “air” without being bright or harsh. The change moved the performance of this DAC up at least two notches. Ever since, I always install this modification in circuits that uses a diode rectifier bridge.
For the high voltage power filtering, I added small value film capacitors to the electrolytic capacitors as by-pass. I used Gen I and II capacitors from Sonic Craft. They are relatively inexpensive for specialty audiophile capacitors and small sized with high voltage ratings. With a recommendation from Brian, I purchased a large Jensen electrolytic capacitor at the time of the purchase as an upgrade to high voltage PSU.
For the Kit1, most of my tweaking was done through changing tubes. I spent quite a bit of time trying out different tubes. I have tried at least 5 to 7 different tubes in each of the positions and the various combinations except for 5687s, a quite a good number of permutations which I repeated once I had a new tube in the mix. Written below are my experiences from this process, the tubes I tried and my impressions about them. . I have been experimenting this for over six years of ownership on Kit 1 so far and I now finally feel more or less content with my selection and the combination of the tube. The valuations were done not just through simple A-B comparisons but having using them for weeks at a time. For the past six years or so while I was searching for the "right” set of tubes, I made no other changes to the amp.
First, a note about 5687 driver tubes
I did not try too many 5687s because the TungSol 5687 which was one of the first ones sounded right from the beginning. I have eventually changed them to a cryogenically treated pair as these retained excellent traits of TungSol sound and added frequency extensions and widen the sound stage. I have briefly tried a Westin House branded cryogenically treated 5687s but the the difference was small and not for the better.
Although this was not the first tube that I experimented with but I decided to write about this first as I now feel that it would had made my tube rolling process easier had tackled this tube first. Further, a choice of rectifier tube had profoundly influence the choices and combination of rest of the tubes in Kit 1.
In the process of tube rolling for my Kit1, I started with 6SN7 followed by300B tubes. As I grew frustrated, someone suggested I should try changing the rectifier tube. Well, little that I knew, it was a beginning of a a quite a bit of a journey. At first, I was very surprised that rectifier tubes do change the sound quite notably. I heard much more difference than changing the driver tubes and at least as much difference as changing the 6SN7 tube. It is a testament to the fact that power supply is critical to the sonic performance of any stereo equipment. I decided to write about this tube first as after I found the one that I liked, the rest of my tube combination changed and I had to test 300B and 6SN7 combination all over again.
I first tried several NOS 5U4Gs in ST shaped glass. Each had its high points but I found that the bass performances were always weak with light and slow. Another thing I noted was that older the vintage, lessor the amount of details. It is quite possible that this can be attributed to individual sample that the age but I did not pursue it further as I did not hear anything special from older tubes that intrigued me.
After trying several 5U4Gs, I put the stock EH 5U4GB back just for a comparison. I was quite surprised how good it sounded and I thought, hmm, I should explore 5U4GBs and its variants. At this time, I did not know that 5U4G and 5U4GBs can be more different than the same (see the note below). May be it was due to the fact that 5U4GB has higher current rating than the 5U4G, the bass was weightier, better defined and kept good pace and rhythm. I tried: EH 5U4GB, RCA 5U4GB; Sylvania5AS4; Sylvania 5931; and cryogenically treated Mullard Australia (labeled as Amalgamated Wireless Valve Co.) 5AS4. 5AS4s are up-rated version and 5931s are ruggedized military spec version. Ultimately I have settled on cryogenically treated Mullard Australia 5AS4 by a small margin. It had the best bass performance with refined and well extended high frequency range without being bright or harsh. If I had not heard this hard to find tube, I would be happy with either Sylvania 5AS4, RCA 5U4GB or Sylvania 5931s. Since even the rarest rectifier tubes are not very expensive, I would encourage readers to experiment before one starts spending on much more expensive 300B tubes.
Note: Emission Lab writes…“There is common misunderstanding that 5U4G and 5U4GB is the same. 5U4GB is a version, with lower internal resistance and higher peak current is allowed. The GB version is not the same tube as the G Version. Replacing 5U4G with 5U4GB may result in higher rectified voltage, so should never be done. Replacing 5U4GB with 5U4G may result in lower rectified voltage, and may result in damage of the 5U4G rectifier.” (http://www.emissionlabs.com/datasheets/EML5U4G.htm)