So … thanks to an unexpected internet-outage, a planned-but-dreaded 2+ hour conference call, and a general desire to cause trouble … the deed is done, at least for the stock-parts build …
It is a super-easy build. I was able to do it while on my conference call, and the entire assembly, including testing and biasing, took a shade less than 2 hours (it’s not a race; take your time). Still not sure I’d want to do it with a 1.6mm tip on my iron though … the 0.5mm I was using was as big as I’d personally want to go (except for the power/RCA/headphone sockets, switch and regulator).
My first actual listening test with it, I used the Grace SDAC and the HD58x Jubilee. And the results were sweet, rich, warm, and clearly imbued with some, quite lush and enjoyable, harmonic distortion products. It might be a good, basic, platform, to illustrate to someone what tubey-sounding-tube-designs sound like vs. pure, inexpensive, high NFB, solid-state designs sound like, as it has a definite and immediately obvious signature.
I had had the thing powered up for probably 20 minutes before first listening (see below on biasing it), and when I plugged the HD58X, even though they were not on my head/ears, the impact of the headphone socket seating home caused enough microphonic ringing that I could clearly hear it. And that took a good 10 seconds or so to fully abate.
That said, with the thing sitting on the desk, while I am typing, even with no music playing the staccato impacts of my mechanical keyboard are not inducing any further ringing or microphones. But if you ever want a demonstration of “tube” microphonics, the above would be a good place to start!
The tube itself looks, as one would expect, like the VFD displays (common in ovens, handheld video games from the 1980s, early calculators etc.) of old. It’s quite charming and I think any design that uses Korg’s NuTube should make it visible at some level.
Yes, I wish it glowed orange (or purple, but tubes, VFD-or-otherwise, don’t do that without LED-assistance), but that’s me.
Some thoughts for potential builders/users:
Definitely do the TDK pot upgrade. Feeding the HD58X, using the SDAC as a source, about 10 o’clock on the pot is as far as I dare turn things up, and it is 9 o’clock before I am only just out of obvious channel imbalance.
For the power LED (D5), the dot on the silkscreen appears to be in the wrong place. For the capacitors, which is what the assembly instructions direct you to follow here, the dot indicates the anode (+). However, the dot for D5 is on the side of the cathode. Tracing the circuit shows that the dotted (square) pad is indeed the cathode. However, if you install the LED in it’s naturally forward-facing orientation you get the proper operation - I’ll ping Pete and see if I’m either interpreting his instructions incorrectly or if they’re actually reversed.
When you get to the point where you bias the two triodes, leave the unit powered up for about 10 minutes before you start. Otherwise you’ll be chasing the slowly drifting values as the circuit (and tube) reaches full thermal equilibrium.
This definitely likes higher-impedance headphones (reasonable given it’s OI). It’s nice with the 58X (150 ohms), better with the 6XX (300 ohms) and leans towards bright with the HE4XX (35 ohms).
So, in short … a super-easy, entirely fun build, that results in a clearly euphonic, and quite pronounced tube-like presentation, for bugger-all money, a minimal investment of time and skill. A Liquid Spark, Magni 3, Atom or Vali 2 likely measure way better (at least in distortion), but none of them have the sweetness or lushness of this little bugger.