It’s no secret that I admire Nelson Pass both for his design skills and for what he gives to the DIY audio hobby. Unfortunately, us vacuum tube enthusiasts are mostly left out in the cold when Pass flexes his design muscles. That is until Burning Amp 2017 when he presented a pre-amp using the Korg NuTube.
Ok, so it isn’t the first “tube” that comes to mind when we think thermionic emission, but hey, it’s got a vacuum at least!
The Korg NuTube is a twin “triode” made by adapting vacuum florescent display technology to audio applications. Just like a DHT, it has an anode, a grid, and a directly heated cathode. The principals of operation (i.e. emission from cathode to anode modulated by a grid voltage within a vacuum envelope) are essentially identical to the little glass bottles we all know and love.
However, the low-voltage miniaturized technology requires certain compromises. With a max dissipation of only 1.7mW, the NuTube is limited in the maximum anode voltage and this forces positive grid operation, requiring a buffer to drive the inevitable low impedance. Like wise, the high plate impedance also necessitates a buffer on the output for most applications.
This is exactly what we see in Pass’s design, using his signature CCS-loaded JFET follower style buffer stage at the input and output. The result is a very compact and relatively low-voltage tube preamp with around 16dB of gain (as designed). Best of all, you can pick up a board and parts at the DIY Audio Store here!
I had the opportunity to hear this preamp not long ago in a nice system (Magnepan speakers, tube and solid state amplifiers) and compared it with an Alex Cavalli designed tube buffer and a MOSFET-based preamp. The Pass preamp sounded fantastic in this good company.
The DIY Audio Store (part of the diyaudio.com forums) is now selling the Elekit TU-8100 PCL86 kit:
At only $275, this 2W output kit is about as low an entry point to all-in-one well-documented tube kits as you’re likely to find. It includes two inputs (rear and 1/8″ front) and is powered by a 12Vdc power brick. This keeps the amp small enough for even desktop usage (5.5″ square). According to the ad copy, SMD components (DC booster, etc) come pre-soldered.
The PCL86 is a 9 pin triode + pentode roughly equivalent to a 12AX7 and EL84 in a single envelope. See a datasheet here. It’s used in the Elekit in a traditional two stage cap-coupled single-ended arrangement. The output transformers are rated as a 7k primary and the circuit employs global feedback to squeeze some extra linearity out of the high gain input stage. See the Mighty Cacahuate for a similar design (no feedback and 6CG7 instead of 12AX7).
If you’re hoping to find a tube kit under the tree this year, add this one to your Christmas list. Elekit puts out some very cool products and purchasing through DIY Audio Store helps support one of our hobby’s precious resources.
I have no affiliation with Elekit or DIY Audio (other than sincere admiration).
Hoffman’s Iron Law impacts all systems, regardless of the type of amplifier. It states that speaker designers may only optimize for two of three performance goals: efficiency, size, and frequency extension. Modern speaker design goals trend towards slim and minimally-intrusive boxes. Because few are willing to give up low frequency ability, this aesthetic trend has resulted in lower-efficiency speakers, requiring ever more powerful amplifiers.
When you have a set of bookshelf speakers or less-efficient towers, a single-ended triode amplifier may not cut it for power. Larger push pull (and parallel push pull) amplifiers are capable of using lower turns ratio output transformers and delivering more power to a load. That could be the difference between realistic dynamics and a more compressed musical presentation. Of course, there are some things to overcome when you upsize your tube amplifier.
The power supply – If building an amplifier with (parallel) push pull power tubes, you’re going to need a lot of heater current. You’ll also need a lot of high voltage current. This means solid state rectification is the way to go. Using a bridge rectifier (four diodes) rather than a full-wave (two diodes) also saves some efficiency in the transformer.
Size and weight – More current demands from the transformer(s) directly translates to a larger size and weight. Again, bridge rectifiers will help reduce the power transformer size slightly. A switching buck converter for heaters is also something worth looking into for efficiency’s sake. Building as monoblocks is a good solution, but you’ll probably spend twice as much on chassis, power supplies, etc.
Current sharing – To keep standing DC currents from saturating the output transformer core, we want all our output tubes to share current equally (or at least balance per phase in each channel). Bias servos, Blumlein garter bias, individual fixed bias, and individual cathode resistors all have their advantages and disadvantages. Careful consideration here is key.
Driving Miller capacitance – With a bunch of parallel output tubes, the input and driver stages will need some grunt to keep Miller Effect from rolling off high frequencies. This is especially true if you’re driving triodes in the output stage. A follower of some type may be needed to ensure a low enough source impedance.
If you haven’t already gathered, I’m a bit preoccupied with how I’ll utilize the big old chassis I picked up recently. Clearly something large is in store. The present question is octals or DHTs and two or four output tubes per side. The chassis originally held some monstrous iron, so there’s space for just about anything.