It's a little frightening that this is still considered state of the art by many manufacturers , and while it is certainly a good sounding circuit, a little imagination can definitely improve the sound by a large margin.
Over the 30 years there have been some constants that I've adhered too, as they majorly contribute to the sound I enjoy. Direct coupling, 6SN7 tubes, tube shunt regulation , silver plated solid core wire, polypropylene or oil caps in place of electrolytic caps whenever possible, "bird's nest" wiring (direct, non capacitive), copper chassis , transformer outputs, are some of the design criteria I've followed for many years. I believe Supratek was the first manufacturer to use DHT as preamp tubes in a commercial sense, the 300B, 45, PX4, 101D were all used in the Supratek range, but I also built preamps using other DHT tubes-the 801/10 series, 26, 30, 33, and the fabulous 71A were all used at some stage, but the preamp tube that has stood the test of time and is probably the most uncompromising of all is the 6SN7.
The Directly Heated Triode tubes are maybe better sound, certainly more fashionable, and definitely harder to work with as it takes a lot of engineering to get them quiet enough for high gain preamp use.
My favorite DHT for preamp use is the low gain 71A used in conjunction with a 6SN7(or 26) as input tube. Transformer coupled it has lots of drive, lots of gain (=dynamics) and suits all amplifiers and types of music.
Unfortunately they stopped making 71A's 70 years ago and they are becoming very hard to find in new condition now.
I have enough to last my lifetime , but not enough for preamp production . The 101D is in current production and with some fine tuning also sounds very good.
It may have been discovered many years ago, there's nothing really "new" in tube design, but a form of tube current setting (bias) was presented in 2004 that is unique and different in concept from the usual forms of setting current-cathode resistors, negative grid voltage,etc.
LowR bias uses a seperate power supply to set the current thought the tube and enables very low resistance values and no capacitor bypassing.
Capacitors are "bad" and minimizing them is always useful, but eliminating them in the bias circuit has very favorable advantages, particularly in regards to phase response behavior . Bass is one area where improvement is noticeable but the overall performance is enhanced. There were a few technical problems to overcome , but the gains were so significant I started to use LowR in my preamps around 2009 , after Kevin Covi brought it to my attention and we evolved the design to fit in with the anode follower 6SN7 design we were playing around with then.
The first thing I noticed about the 6SN7 circuit with the LowR bias was the similarity of the sound to my DHT preamps, indeed in some systems I even preferred the sound of 6SN7 LowR to the conventional 71A preamp.
At the price we can build the lowR 6SN7 preamp I believe it is probably the best performing preamp in the world.
Of course we incorporated the LowR concept into the DHT preamps, and after quite a lot of experimenting, and some really wild circuits that were very intriguing and interesting to build, we settled upon a DHT preamp design that I believe is unique and unlike anything else available at the moment. The bass performance is extraordinary, 3D imaging is startling, and it brings any system to life.
There is increasing talk about the role of analog preamps disappearing, I think this is rubbish, and anyone who hears a great tube preamp in a digital system will instantly recognise it as an absolute neccesity.
6SN7-71A preamp
