300B Dual Cabernet

Just finished this 300B Dual Cabernet, and very happy with this one. I haven't built a 300B preamp for quite a while, and incorporating some new design ideas and the features of the "dual" preamp with the iconic 300B tube turned out very nice indeed.

I don't think many people "get" the concept of the DUAL CABERNET so I've copied some of the text from user manual that explains further. Its from the 4P1L manual but also applies to 300B or any DHT tube used.

"The Dual Cabernet has two separate line stages, one utilising a Directly Heated Triode tube – the 4P1L , and the other line stage utilises a two stage tube design , using one half of the dual triode 6H8C/6SN7 and a 6N6/6H30. These two tubes are Indirectly Heated Triodes, which means they have a separate heater , rather than the directly heated filament of the DHT tubes.

Both stages can be used independently , or together. The gain of the 4P1L has two settings, and the gain of the 6N6 circuit can be set with it’s own adjustable level control.

Overall gain is set by the master volume control on front panel.

Both circuits, the DHT and the IDHT are state of the art designs, with high bandwidth, very accurate frequency response, and excellent drive ability.

The 4P1L has all the attributes of DHT tubes, a sense of “tube euphonics” , some microphonics (not a bad thing) and a pleasing, elegant musicality.

This stage also has the option of switching in the other half of the 6H8C/6SN7 triode, which will give higher gain, useful for using with low sensitivity amplifiers, or very low efficiency speakers.  You may find a greater sense of dynamics when this driver stage is switched in, depending on your system and requirements.

The Dual Cabernet’s 4P1L line-stage has a switchable output to choose either transformer or direct output. The transformer output, with switch towards rear has lower gain and because of it’s lower output impedance more drive, handy when driving multiple amplifiers , or a difficult load.

The 6H8C/6N6 circuit is simply one of the most technically advanced tube circuits available. It has very high bandwidth, extremely low distortion, very low output impedance, which enables it to drive any amplifier. It is a “state of the art” design using some of the best audio tubes ever produced.

With both of these circuits you have the very best of both the DHT and IDHT worlds, with this preamplifier you are sure to be able to find a combination that finds the perfect synergy for your system and tastes.

One way to use this dual preamp is with bi-ampable speakers, with separate connetions for bass and mid/treble.

The 4P1L linestage would be connected to the amplifier driving mid/treble and the 6N6 linestage connected to amplifier driving bass. Or the opposite, depending on your requirements, taste etc.

The 6N6 level control can then be set to give the best balance of bass and mid/treble to get exactly the tone preferred for your room and system."

The 6SN7/6H30 IDHT line-stage  alone would cost far more than the total cost of a DUAL CABERNET if purchased from a mass produced high-end audio company, but with the DUAL you get a world class DHT line-stage as well, and a preamp that is custom hand made , tested , tuned and run for at least 3 days before sent to owner.

There's nothing else like it in uniqueness and sound quality in this world.

6SN7/6H8C/6F8G

I've been using the double triode 6SN7 tube for as long as I can remember, I have a collection of about 500 NOS 6SN7's , about the same of the single triode version the 6J5.
The older NOS tubes sound damn nice , but they are frequently microphonic, and although I don't think that is necessarily a bad thing, with a high gain preamp that can be a problem.

NOS 6SN7's 

For at least 10 years I've been using the Russian 6H8C version of the 6SN7, it is a well manufactured, sturdy version that sounds very good, and is not too microphonic.

Russian 6H8C

Another earlier version of the 6SN7, made around the 1930's is the 6F8G. This has a top cap for one of the grids, which is a bit inconvenient as a hole has to be made in the chassis for the top cap connection.  Although it uses the same octal socket as 6SN7, the pin connections are quite different.
But its a fine sounding tube, and I built a few preamps and power amps that used it.
I built one power amp with 6F8G and 807 , both using top caps which gives an "antique" look.
Recently there have been some convertors available on Ebay which allows one to use the 6F8G in place of 6SN7 without any re-wiring of circuit.
One of my Russian customers asked if it was worthwhile, so I bought a pair to give it a try.
They look kind of stupid, but there is something about the 1930's period tube sound  that is very nice, it's different rather than better, but it is a enjoyable alternative , and the 6F8G tubes are relatively inexpensive, or they were when I last bought some many years ago.
A fun experiment for those of you with 6SN7/6H8C preamps.