uTracer NXT — Vol 4: Building the Kit & Host Software

4.1 What arrives in the box

The NXT follows the uTracer kit tradition: a through-hole PCB, a bag of components, a pre-programmed PIC16F884, and documentation, meant to be hand-soldered by the builder. It does not ship as a finished, cased instrument, and it is not surface-mount reflow work — the deliberate through-hole choice (Vol 1) is what keeps it buildable at a bench with an iron. You supply the enclosure, the tube sockets/wiring to suit the valves you test, and a ~20 V DC brick to power it — the same class of laptop-style supply the earlier uTracers used, which is a big part of why the instrument stays small and cheap (no mains transformer of its own). The board is dimensioned to the uTracer3 footprint and terminal layout, so builders coming from a 3+ will find the mechanical layout familiar.

Caveat on build specifics. The NXT build log is an evolving engineering journal, and a formal step-by-step NXT construction manual of the kind that exists for the 3+ and 6 is TBD — confirm against what actually ships with the kit. The uTracer3+ and uTracer6 each shipped with an extensive printed construction manual; expect the NXT to follow suit, but the exact document and revision are not something to quote from the build log yet.

4.2 Assembly overview

The order of work mirrors the earlier kits and the natural signal-chain grouping (Vol 2):

• Low-voltage/power section first. Fit the +5 V regulator (78L05-class) and confirm the logic rail before anything else goes in — the PIC, DAC, and PGA all depend on it. Confirm the −105 V grid rail’s small boost converter comes up.
• Passives, then the analog front end. Sense resistors (the 14.3 Ω shunts), the MCP6V86 buffers, and the PGA113 programmable-gain stage. A build note from the log: a bypass capacitor directly across the PGA’s supply pins is needed for clean readings — a detail worth honoring exactly as specified.
• HV generation. The 330 µH boost inductors (mounted vertically on pin-header stubs, per the log, for easier soldering and board fit), the boost NMOS, the diodes, and the 100 µF / 500 V reservoir caps. The 500 V reservoir caps can be a long-lead-time part; 450 V alternatives are accommodated at a slightly reduced envelope.
• HV switch and grid amplifier. The NMOS high-voltage switch and the OPA455-class grid amplifier (the log used sockets for the DAC and HV op-amp in the prototype, which is convenient for a first build).
• Firmware/serial. The PIC arrives programmed; you verify communication over the 9600-baud serial link with the host before doing anything with high voltage.

Standard HV discipline applies throughout, and it is written into this project’s bench notes: the reservoir caps hold charge after power-down, so verify the bleeder has done its job before opening the unit, and treat the tube socket as live during a sweep.

A couple of small-but-real build details surface in the log and are worth honoring exactly rather than second-guessing: the boost inductors are mounted vertically on header-pin stubs (better fit and easier soldering than lying flat), and the PGA supply bypass cap goes directly across the chip’s power pins — the log calls it out specifically because the programmable-gain stage is sensitive to supply noise and misbehaves without it. When a one-off note like that appears in an engineer’s journal, it is usually there because he was bitten by its absence.

4.3 Calibration

Because the NXT measures by digitizing small voltages across real resistors and amplifiers, it must be calibrated once so the host can convert raw ADC counts into accurate volts and milliamps. The build log describes a software-driven calibration with two tiers of user interface (the same philosophy as earlier uTracers):

• Slider (“novice”) mode, where each channel has a calibration multiplier you nudge over a narrow range (roughly 0.9–1.1) until a known setpoint reads correctly.
• Direct (“expert”) mode, where you type the actual measured resistor/divider values so the host uses the true hardware constants — this is also how you tell the software about any range-extension mods you fitted.

The grid channel gets the most attention because it must be accurate from −100 V all the way down to a fraction of a volt. The log describes a small multi-point procedure: null the amplifier’s offset (adding a small correction resistor if the offset sits the wrong side of zero), set the slope using a large bias point (e.g. −50 V), then trim the offset term using a very small bias point (a couple hundred millivolts), aiming for roughly <1 % error over −100 V to −1 V. The heater channel is calibrated against its PWM duty-cycle-to-voltage relationship (voltage tracks the square root of the duty setting). Calibration constants are stored by the host and reloaded on restart, so it is a one-time exercise per build.

4.4 The host software / GUI

The PC-side application is where you actually operate the instrument (Vol 5 walks a session). From the build log and the uTracer family precedent, the GUI provides:

• Measurement-type selection — pick what to sweep and what to hold (e.g. sweep Va with Vg stepped between curves, heater held constant).
• Auto-gain and auto-averaging — the software picks the PGA gain that fits each point and can average several pulses per point for noise.
• Compliance / current-limit setting, scaled to the selected current range, so a shorted or runaway tube aborts the pulse instead of stressing the hardware.
• A soft heater ramp, issuing a sequence of increasing heater setpoints rather than slamming a cold heater to full voltage.
• Live on-screen plotting of the curve family as points come back.
• The two-tier calibration form described above.

One honest gap: the build log does not fully pin down the NXT host application’s language/platform or its exact feature list — the earlier uTracers used a Windows GUI, and the NXT is expected to continue that lineage, but treat the specific application details as TBD — confirm against the software that ships with the kit rather than assuming.

4.5 Data output

The end product of a session is the curve family plus whatever you export for downstream use:

• On-screen curves — Ia (and Is) versus Va, one trace per grid-bias setpoint, drawn live.
• Tabular / CSV-style export — the uTracer toolchain exports the measured points for use in spreadsheets and plotting tools. The NXT-specific export format is not spelled out in the build log yet, so treat the exact CSV columns as TBD — confirm.
• SPICE models — generating simulate-able tube models from measured curves is a longstanding goal and workflow in the uTracer ecosystem. Whether/how the NXT host automates SPICE-model extraction specifically is not documented in the build log at this time (TBD — confirm); don’t assume a one-click NXT SPICE exporter until the shipped software confirms it.

The safe summary: the NXT will give you live curves and exportable measured data; the finer export/SPICE details are the part of the toolchain still being written up, and this dive will get its bench-confirmed answers once Jeff’s kit is built and running (Vol 5).