uTracer NXT — Vol 5: Using It + Generation Comparison

5.1 The operating workflow

Using the NXT is a rhythm of setup, sweep, read, export. In practice:

1. Wire the tube. Connect the valve’s pins to the anode, screen, grid, cathode, and heater terminals for its socket. Get the pinout right — this is the step with live HV consequences.
2. Power and connect. Apply the ~20 V brick, open the host GUI, and confirm the serial link (a “ping” returns the supply voltage). Nothing high-voltage has happened yet.
3. Light the heater gently. Set the heater voltage for the tube and let the software’s soft ramp bring it up rather than cold-slamming it. Let the cathode reach temperature before measuring.
4. Define the sweep. Choose the measurement type — for a triode plate-curve family, sweep Va across the range and step Vg between traces; hold the heater constant. Set the current compliance to protect the tube and the instrument.
5. Run. The NXT executes the pulsed cycle (Vol 3) for every point: charge the reservoir cap, fire the ~1 ms pulse with the grid biased, sample Ia and Is, discharge, advance to the next point. Curves build on screen live.
6. Read and export. Inspect the family, read off or let the software derive parameters, and export the measured points for archiving, plotting, or modeling (Vol 4 — some NXT export specifics are still TBD).

The pulsed nature means a full family is not instantaneous — each point includes a cap-charge phase — but it is unattended once running, and the tube stays cool throughout.

5.2 An example measurement (illustrative)

Say you’re characterizing a small triode. You set the heater to its rated value, choose “Ia versus Va, Vg stepped,” and tell the software to sweep Va from 0 to ~300 V while stepping Vg from 0 V to −8 V in 2 V steps, with compliance set to a safe current for the tube. The NXT returns five traces — one per grid setpoint — that fan out exactly like the illustrative figure above: at Vg = 0 the tube conducts hard and the curve rises steeply; as the grid goes more negative each curve shifts right and flattens. From that family you read the classic parameters at any operating point: the vertical spacing between curves at fixed Va gives transconductance gm, the horizontal spacing at fixed Ia relates to the amplification factor μ, and the local slope gives plate resistance rp. Then you export the points. (The numbers here are an illustration of the procedure; they are not measured NXT data — Jeff’s kit isn’t built yet.)

5.3 Where the NXT fits on Jeff’s bench

The NXT is the everyday, publishable-curve tracer: modern, parts-available, robust, in the ~500 V / few-hundred-milliamp band that covers the overwhelming majority of receiving and audio tubes. The uTracer6 remains the tool to reach for when a tube genuinely needs kilovolt plates, an amp of current, or positive grid drive (RF transmitter finals). The quick go/no-go testers on the bench — the Heathkit TT-1 (Gm) and the Supreme 385 (emission) — handle triage; the tracers produce the curves and models.

5.4 uTracer3+ vs uTracer6 vs uTracer NXT

This is the comparison Jeff asked for. Figures are drawn from the dos4ever build logs; where a value is a target, a demonstrated peak, or a mod-dependent extension, it is flagged. Treat ”≈” figures as build-log-current and re-verify against the shipped kit before relying on the last digit.

Headline comparison of the three generations:

Table 1 — Headline comparison of the three generations:

Aspect	uTracer3+	uTracer6	uTracer NXT
Role	Mass-adopted hobby tracer	High-voltage / RF-tube specialist	Modern redesign of the mainstream tracer
Introduced	Jan 2015 (3+; V3 ~2013)	~2020 (HV variant)	2020s (build log ongoing)
Anode/screen voltage	0–400 V	up to ~1000 V (target ~800 V)	≈2–500 V (with 500 V-rated caps)
Anode/screen current	~200 mA (trip ~220 mA)	up to ~1 A	≈350 mA default (≈750 mA with sense mod)
Grid bias	0 to −50 V	0 to −100 V (and 0 to +100 V + grid-current meas. on optional extension board)	≈0 to −100 V (negative only)
Heater	0–19 V	as uTracer3, lower PWM freq	0 to ~supply, ~1.2 kHz PWM
Pulse width	~1 ms	~1 ms (10 µs software mode)	~1 ms
HV switch device	HV PNP transistor	NMOS (1000 V class) + push-pull driver	NMOS (700 V class, ~IPD70R360P7) — 6’s topology
Boost switch	—	SiC MOSFET (SCT2750NY, 1700 V)	NMOS boost, 330 µH inductor
Sense op-amp	OPA227	OPA227	MCP6V86 (zero-drift) + PGA113 PGA
Sense resistor	~18 Ω	4.7 Ω (for ~1 A)	14.3 Ω
Grid DAC	— (family DAC)	DAC8562 (dual 16-bit)	MCP4921 (12-bit) + OPA455 HV amp
Microcontroller	PIC16F884	PIC16F884	PIC16F884
ADC	10-bit	10-bit	10-bit (PGA-ranged)
Interface	Serial (via USB adapter)	Serial (via USB adapter)	Serial, 9600 baud (via USB adapter)
Auxiliary rails	±15 V auxiliaries	—	eliminated (+5 V and −105 V only)
PCB	10 × 16 cm	6 × 6 in (152.4 mm)	uTracer3 footprint (drop-in layout)
Kit form	Through-hole kit	Through-hole kit	Through-hole kit

Reading the table — the three things that actually changed in the NXT:

• The high-voltage switch moved from PNP to NMOS, adopting the uTracer6’s field-proven topology (but at a 700 V-class device suited to the NXT’s ~500 V envelope, not the 6’s kilovolt parts). This is the reliability upgrade.
• The analog front end was modernized around available parts — the obsolescing OPA227 gives way to a zero-drift MCP6V86 plus a PGA113 programmable-gain stage, which also improves range handling. This was the reason for the redesign.
• The power architecture was simplified — the ±15 V auxiliaries are gone, leaving a clean +5 V / −105 V scheme.

And the things that deliberately did not change: the pulsed ~1 ms measurement principle, the PIC16F884 core, the 10-bit ADC, the serial interface, the through-hole hand-solderable kit form, and the ~500 V-class envelope that puts the NXT squarely in the popular-tracer band rather than the uTracer6’s specialist kilovolt territory.

5.5 Open items to confirm on the bench

Because the NXT is newer and less documented than its siblings, several items in this dive are flagged TBD — confirm and should be nailed down once Jeff’s kit is built and registered under MY_GEAR slug utracer-nxt: the exact host-software application and its feature list, the CSV/export column format, whether the NXT host automates SPICE-model extraction, the formal construction manual and revision that ships with the kit, and the last-digit precision of the envelope, trip points, and range-extension component values. Everything in Vols 2–3 about the architecture and the pulsed measurement theory is well supported by the build log; the downstream software and the shipped-kit particulars are the parts that firm up after the build.