A Black Box to Solve Everything

Nelson Pass's DIY Nutube preamp kit allows for adjustment of distortion. Here is a description from the design document:

"The attraction of the Nutube in an audio circuit is the Triode character, allowing adjustment of the distortion by operating the device in a specific voltage/current load line. For a given amount of current through the tube there is a point at which the square law effects of Cathode current and Plate voltage cancel, nulling out 2nd harmonic distortion and leaving 3rd harmonic. This is the lowest output distortion setting.

If you increase the Plate voltage, the circuit takes on a positive phase 2nd harmonic distortion character, and if you lower the Plate voltage it has negative phase 2nd harmonic. These distortions increase as you adjust the Plate voltage farther from the null point"

@bdp24 Yes a preamp and a reasonably easy kit to build. Nelson is brilliant with solid state devices and while Ralph and Roger are/were known for their brilliance with tubes, Ralph has now demonstrated he can manufacture a solid state amp with similar distortion characteristics as tubes. Somewhat of a little known fact, Roger's first amp to be manufactured was the one he designed for the Beveridge 2SW speaker. It was a solid state amp designed specifically for the woofer section. This required him to develop the RM-3 active crossover to act as a frequency dividing network as the 2SW required biamping. The RM-3 also used sold state devices.

Getting back to the B1, Nelson's choice of the Korg Nutube is interesting and I'm sure was selected to support his circuit requirement to allow for the adjustable harmonics. However, it's probably the worst tube I have ever dealt with. Many examples of this tube are extremely microphonic. I went through 3 of them building that kit and none of them were quiet enough. So I never could really test out the adjustable distortion feature.