The Nightwalker Bass Guitar Tube Preamp is intended to bring back the old school bass tones that dominated the airwaves decades ago. All user controls and jacks are conveniently located on the front panel, which includes a single input and a pair of outputs: one labeled “High Freq” and another labeled “Low Freq/Mono.” Each of the outputs is driven by a respective half of a single 12AX7 tube.There is a volume control associated with the high frequency output and a volume control associated with the low frequency output. A switch roughly centered between the volume controls and output jacks is identified as a bi-amp switch. There is also an overdrive switch and a footswitch input jack. All in all, this is a set-and-forget style of preamp. You will not waste a bunch of time trying to dial in your sound. Plug in, turn up and groove.

Construction

The Nitewalker preamp is in a “brick” style enclosure. The unit is not rack mountable, but the housing is dimensioned so as to be easily placed on top of an existing rig. Taking a look at Fig. A, all user controls are panel mounted to the front panel and are hand-wired to the circuit boards. A power cord and a fuse extend from the back panel. The result is a clean minimalist layout. A pair of circuit boards stand mounted back to back, in a vertical orientation thus dividing the inside of the enclosure into two sections.The right-side section (looking from the front panel) houses the preamp circuitry and a transformer for the power supply. The left-side section (looking from the front panel) includes a large choke for the power supply and the power supply circuitry on a daughter card that is coupled to the preamp circuit board by brass standoffs.

Taking a look at Fig. B, the preamp circuitry is soldered to a single circuit board that spans the entire height and depth of the enclosure. The circuit design takes advantage of a blend of vintage style capacitors and modern capacitors. The single tube plugs into a nice quality socket that is soldered directly to the preamp circuit board. I pulled the tube from the socket and reinserted the tube numerous times and could detect no play or give.

Overall, the circuitry and construction inside the enclosure takes advantage of quality modern and vintage parts. The potentiometers have a nice feel to their turn, and the knobs are easy to hold and adjust.If I had one complaint, I would prefer nicer quality jacks for the input and footswitch.

Circuit Layout

The Nitewalker preamp is a straight-forward circuit that provides a single tube gain stage and some tonal shaping from a hand-full of passive components.However, there are a few twists that set this design apart from conventional fare. First off, this circuit is not a traditional bi-amp circuit. Rather, as seen in Fig. C, the “high” frequency channel and “low” frequency channel share a similar low end response, but the high frequencies are extended slightly on the high channel. Both the low and high frequency channels include a vintage capacitor in a high frequency compensation network at the anode of the tube stage that participates in the gradual treble attenuation. However, the low frequency channel adds a second, series vintage capacitor, which the designer credits as being a key contributor to the difference in character between the two channels.

Both channels use a single stage of a 12AX7 tube.As such, each output is inverting, relative to the input, as seen in Fig. D. Note that in Fig. D, the input signal is the black trace. The high frequency output channel is the red trace and the low frequency output channel is the blue trace. The outputs are adjusted to unity gain. However, there is a slight amount of DC leak that shifts the response down slightly. The DC shift is slightly more pronounced in the low frequency output channel. The slight amount of DC shift varies depending upon the position of the biamp switch due to the interaction of the two channels when the biamp switch is in the up position.

One feature that I find interesting about this design is its use of grid-leak biasing of each tube stage. This is certainly not new, and was seen most frequently in tube designs of the 1940s and early 1950s. Grid-leak biasing has an interesting property, in that the biasing point tends to shift as the input signal level fluctuates. For instance, in our testing, the grid voltage varied from -0.6 Vrms to about -1.6 Vrms, depending upon the level of our test input signal.

What does this mean to the performer? The bias voltage affects how the tube performs. Without getting too techy, the user will perceive this as a compressor/limiter-like effect that contributes to the overall tone of this preamp. There is no dedicated limiter circuit in this preamp. However, compression-like effects can result because the tube stage is biased towards and into cutoff for large input signals, causing asymmetric gain leading into positive edge limiting.

Effects of Grid-Leak Bias

To demonstrate the effect of this biasing scheme, take a look a Figs. E-H. We applied a steady state input signal at 100 mVrms, 400 mVrms, 600 mVrms, and 1 Vrms on the bass channel. The volume knob was adjusted to provide about unity gain at our 100 mVrms input and the volume knob was not adjusted for the remainder of the test. Note that as the input level is increased, the positive half of the output is compressed and limited. This shows the effect of the grid-leak bias scheme driving the tube into cutoff. As these charts illustrate, the output level of the bass guitar will strongly shape the character of the output.A mild output bass will exhibit less compressive effects. Likewise, a bass guitar with a strong output can push the circuit into deeper asymmetrical output.Whether that is a good thing or a bad this is purely a matter of subjective opinion.

The effect of the varying bias is also likely the culprit that contributed to the preamp causing a bit of instability when testing with the Audio Precision test analyzer. For instance, a steady state input of 630 mVrms caused the frequency sweep to become unstable and to ring as seen in Fig. I. Now, keep in mind that a bass signal is not a steady state signal, like the test signals herein. Where a bass may exhibit a peak up to or even exceeding 1 V, the envelope of the bass signal will settle down to a level well below the peak, and will typically be below 630 mVrms. I could not detect any such ringing or other similar effects when playing the preamp with an actual bass.

The biamp switch basically controls the interaction between the two channels. In a first position, each channel is directed to its own jack. In the opposite setting, the two channels are mixed together and the signal is output to the bottom jack. As such, the controls are interactive.

The footswitch controls the overdrive mode of the preamplifier. The overdrive mode can also be controlled by a switch on the front panel. Again, this is an interesting implementation. Because each channel is a single tube stage, grounded cathode, grid-leak biased configuration, the overdrive is implemented by changing the value of the anode resistor to increase the gain of the tube stage.

Because the tube stage is biased closer to cutoff, the distortion experienced is largely due to asymmetrical clipping.

Referring to Fig. J, the distortion is plotted as a function of input signal level. For this test situation, and as this chart illustrates, the preamp heavily weighs towards coloring the sound. The lowest distortion occurs with input levels below 10 mVrms, with distortion levels reaching 5% with an input signal just over 100 mVrms. A signal just under 1/2 Vrms can produce in excess of 40% distortion. Note that this is completely consistent in what we saw above with the asymmetric clipping of the positive cycle as the preamp tube pushes into cutoff. Again, this is the character of grid-leak biasing.

Handling a Burst to the Input Signal

Referring to Figs. K-M, an input signal was applied that burst at full volume and then dropped 20dB. The input burst at full volume was 100 mVrms, 250 mVrms and 500 mVrms, respectively. The tube stage causes the overall response of the preamplifier to be “loose,” not tracking the input level perfectly.Rather, the asymmetry of the tube stage causes a slight compression that can be clearly seen in the envelope of the output of the preamp when compared to the input signal (in black). This effect gets progressively more dramatic as the input level increases, which is again, consistent with the character of a grid-leak biased tube stage.

Conclusion

Keep in mind that this is a preamp only. As such, you will need to plug this into a power amplifier. The amp you plug this into can thus affect how you perceive its tone. Also, the grid-leak bias of the tube stage can provide a wonderful compressive style of response. Alternatively, it may not suit your playing style. This is a subjective determination that you will have to make. Also, you may be able to “play” the preamp by controlling your overall volume to take advantage of the compression-like effect of the tube stage.