Vacuum Tubes or Purist Tone Control?

Area for experimental notes regarding valves used in the Concert Fidelity DAC 040, and the CF 080 LSX2 Pre Amplifier.

Both components utilise a EZ81 / 6Ca4 full wave rectifier, and 12au7 and their many variants, in the analogue output stage. Tubes, valves, triodes, whichever you prefer, are accessible on the rear panel, for both pieces. Reasoning behind exposing the analogue output stage tubes here is two-fold: firstly the circuitry is kept to an absolute minimum in length, maybe the shortest signal path in any active audio component ever made, with the output stage mounted vertically just inside the rear panel, allowing for a signal path length as low as 11cm! This is clearly contributing to the totally transparent sonic signature of the output stage. The tube sockets are mounted towards the lower edge of the board providing safe access from the rear of each unit. Secondly, and most importantly for the tube roller, is that the sonic character of the tube fitted is what you will hear, the circuitry design is so minimal that it has next to no sonic influence upon the signal itself, thereby careful tube selection will alter tonality to suit the music, the room, the loudspeaker system even. The tubes are protected from accidental damage by  four solid metal barriers, and are a very precise fit into the sockets, they are a firm and secure fit, with excellent contact to the socket. The tubes also do not get excessively hot either, helping those favourite triodes last even longer..

Some people assume that vacuum tube amplifiers will all sound ‘warm’, maybe a little soft even. Well, no, that is not the tubes doing that, it is inferior circuit design. When the circuit is designed to minimalist standards, the tube sound is as crisp and finely etched as any solid state design, but without the hard edge present in so many SS components. This may take a designer many years to perfect, and Tsuda-san that has produced these items built his first working vacuum tube amplifier when he was just ten years old. Yes, you read that correctly, no typo…


To date, we have used:

Mullard, Tungsol, Mazda, Sylvania, JJ, Miniwatt Dario, Tesla, RSD, Tungsram.

Variants include:

12au7, 6189, ECC82, ECC802s, 5814, 12bh7, 7316.

6Ca4 / EZ81 Rectifier Tube

Mullard 6Ca4 is one of my favourite in this position, it has just perfect balance, precision, texture – a very organic sounding tube. It is so natural, and recreates the sense of space that is very addictive. It does not in any way sound soft or too warm, or on the other hand lean. Simply wonderful.. I am steadily putting together a (Nos) collection of these whenever I see them available, for use with the Concert Fideleity DAC 040, and 080LSX2 Pre-amplifier.

Mullard 6Ca4 Rectifier Tube.

Another tube we use in this position, is the Mazda EZ81, this maybe has slightly more body than the Mullard, but again, not overly so, perhaps a bit punchier too. A lovely tube without doubt, less common to find.

Nos Mazda EZ81

Also very nice is the Amperex, a similar balance to the Mullard, very clean and precise, perhaps not quite having the palpability of the Mullard, but still very good.

Output Stage Vacuum Tubes – 12AU7

Used in both the 080LSX2 Pre-amplifier & DAC040 at the output stage, the tonality of this tube is very evident on the overall sonic signature of the system the pieces are used in. Never before has the character of a tube been so evident to our ears at least, remember the days when tone controls were removed by purist designs back in the late seventies maybe? Well, here is the ultimate tone control, of the purest form without all the degrading circuitry!

So much choice, where to begin? The 12AU7 is the US label for the European ECC82, same tube, different manufacturers. With that comes subtle differences in tonality, FR, soundstage, crispness, front to back depth etc, meaning that a desired presentation can be dialled in using a particular balance of tube choice. Of all of the triodes that we have used, none have cost a significant amount, all are NOS, (new old stock), none are the exotics such as Telefunken ECC802s, some offer a more punchy presentation, some more refined. This leads us nicely on to application, for here we have the option of a quick swap to a tube pair that one can select for a particular genre, or maybe even a particular recording, as we have done. We found that the Tesla ECC802s for example gave such a clean and refined presentation, we just love this tube for classical recordings, above most others, it seems so well suited. Another example was when we installed a pair of 12BH7 into the DAC 040 for David Gilmour’s ‘Live in Gdansk’, the slight difference in tube properties on this album were immediately apparent and really drove that recording to its limit! But, with classical, I personally felt it was a bit thick, congested perhaps, the cleaner ECC82/802s being preferable.

We have found some favourites, or favourite combinations is probably a better term, as sometimes using the same tube compliment in both the DAC & Pre does not equal better results. I will expand this list as I discover more, but will start with what I know now..

Mazda 6189 (12au7WA)***** French made. Nos, date unknown.

A star, one I have come to regard as a best friend, always trusting what will happen when used in both pre & DAC. Great dynamics, refined and clean, with wonderful tonality, it is becoming a very good choice for all round use, very natural.