uTracer NXT — Vol 1: What It Is & Why It Exists

1.1 The one-sentence version

The uTracer NXT is Ronald Dekker’s modern, ground-up redesign of his long-running DIY pulsed tube curve tracer, published on the build log at dos4ever.com/uTracerlogNXT. It keeps the idea that made every uTracer distinctive — pulse the high voltage into the tube for about a millisecond instead of supplying it continuously — but rebuilds the electronics around modern, still-available parts, folds in the proven high-voltage switch from the uTracer6, and stays a through-hole hobby kit that drops onto the original uTracer3 board footprint.

This volume sets up the lineage and the “why.” Vol 2 opens the hardware; Vol 3 covers the measurement theory; Vol 4 is the build and the host software; Vol 5 is operating it, with the head-to-head comparison table Jeff asked for.

1.2 Why a curve tracer at all, and why pulsed

A tube curve tracer sweeps a set of controlled voltages onto a valve — plate (anode), screen grid, control-grid bias, and heater — and records the resulting anode and screen currents. The output is the family of I–V curves you find in a datasheet, plus derived parameters (transconductance gm, amplification factor μ, plate resistance rp). You use it to match tubes, grade used stock, and generate SPICE models for amplifier simulation.

The obvious way to build one is a bench full of hefty regulated HV supplies, big heatsinks, and fans, because a power tube at a few hundred volts and a few hundred milliamps dissipates real wattage continuously. Dekker’s founding insight — the thing that made the original uTracer tiny and cheap — was to avoid all of that. Instead of holding the plate at, say, 300 V forever, the uTracer charges a reservoir capacitor and connects it to the tube for only ~1 ms, samples the current at the end of that pulse, and disconnects. Average power stays trivially small even though the instantaneous operating point is a real one. That is why a uTracer needs no transformer bank, no heatsinks, and runs from a laptop brick. The pulsed principle is the through-line across all four generations, and Vol 3 works through the physics of it.

1.3 The lineage, generation by generation

uTracer3 / uTracer3+. This is the one that made the design famous — the mass-adopted hobby kit, sold in the thousands across dozens of countries over more than a decade. The original V3 covered roughly the 0–300 V region; from January 2015 the uTracer3+ replaced it with an extended 0–400 V anode/screen range. Practical envelope: about 200 mA of anode/screen current (a hard-wired over-current trip around 220 mA), grid bias 0 to −50 V, and a heater supply 0–19 V. It runs from a 19.5 V laptop supply, talks to a PC over a serial link, and lives on a 10 × 16 cm board. For the great majority of receiving tubes — preamp triodes, small-signal pentodes, common output tubes at sane operating points — the uTracer3+ is entirely sufficient, and that is exactly what Dekker argues: most interesting tube parameters are well characterized between 0 and 400 V.

uTracer6. The “6” was the high-voltage answer to the one thing the 3+ could not do: big RF transmitter tubes and high-voltage audio finals. Dekker set target specs around a 6146B transmitter tube — anode/screen up to ~1000 V, currents up to ~1 A, and, crucially, positive as well as negative grid bias (0 to −100 V, plus 0 to +100 V with grid-current measurement on an optional extension board). Getting there forced two hard engineering problems to be solved: generating and switching a full kilovolt cleanly. Dekker moved the boost converter to series inductors driven by a silicon-carbide MOSFET (the SCT2750NY, 1700 V), and — the part that matters most for the NXT — replaced the fragile high-voltage PNP switch of earlier designs with an NMOS high-voltage switch using 1000 V devices and a push-pull gate driver. That switch proved extremely robust in the field. The 6 lives on a 6 × 6 inch (152.4 mm) board. Early on Dekker pushed an EL34 to roughly 900 V and 900 mA — nearly 1 kW — under pulsed operation to prove the concept.

uTracer NXT. The NXT is neither “3+ with more voltage” nor “6 made cheaper.” It is a modernization, and the trigger was parts obsolescence rather than a new performance target. Two components the earlier designs leaned on were disappearing: the OPA227 op-amp in its through-hole DIL package, and suitable high-voltage PNP transistors. Dekker had already prototyped a fix — a tiny adaptor board that drops a modern MCP6V86 zero-drift op-amp into the OPA227’s socket — but rather than paper over an aging design he took the opportunity for what he describes as a more modern and future-proof overhaul, while deliberately keeping it through-hole so hobbyists can still build it by hand.

1.4 What problem the NXT actually solves

Three, really:

• Longevity of the design. By replacing the obsolete OPA227 and HV PNP parts with current-production devices (the MCP6V86 sense amplifier, a PGA113 programmable-gain stage, and an NMOS high-voltage switch in place of the PNP), the NXT can keep being sold and built for years without a scavenger hunt for dead parts. See Vol 2 for the part-by-part swaps.
• Bringing the 6’s robustness down to the mainstream instrument. The NMOS high-voltage switch that survived thousands of field pulses in the uTracer6 is carried into the NXT, so the everyday ~500 V-class tracer inherits the reliability advance that was previously only in the kilovolt model.
• A cleaner, simpler platform. The redesign collapses the old auxiliary ±15 V supplies into a simpler rail scheme (a single +5 V logic rail plus a −105 V grid rail), and reorganizes the analog front end around the PGA — modernizing resolution and range handling without abandoning the parts-on-a-through-hole-board ethos.

Where the NXT sits in the envelope is deliberately in the 3+ band, not the 6’s: roughly up to ~500 V on the plate and a few hundred milliamps (see Vol 2 and the Vol 5 table for the exact figures and the range-extension mods). It is the successor to the popular tracer, not a replacement for the specialist kilovolt one. In Dekker’s own framing, most of the tubes people actually test are comfortably characterized inside that window.

1.5 Who built it, and where the record lives

The whole uTracer line is one designer’s work: Ronald Dekker, publishing at dos4ever.com. That matters for how you read the sources. There is no marketing department and no glossy spec sheet — the primary record is a build log, an engineer’s running journal of decisions, dead ends, and measurements. The uTracer3+ and uTracer6 logs are mature and have matching printed construction manuals because thousands of those kits are in the field; the NXT log is still being written as the design settles. So the authoritative NXT source is the NXT build log itself, cross-checked against the uTracer3 and uTracer6 logs for the baseline this dive compares against. This is exactly why later volumes flag some downstream details as not-yet-published rather than guessing — the journal simply hasn’t reached them.

1.6 A note on documentation maturity

The NXT is genuinely newer and less documented than the 3+ and the 6. The build log is an evolving engineering journal, not a finished manual, and several downstream details — the exact host-software feature set, the CSV/export format, and the SPICE-model workflow for the NXT specifically — are not yet fully published. Where a fact isn’t confirmable from the build log, this dive says so rather than inventing a number. The hardware architecture and measurement theory, by contrast, are well covered, and that is where Vols 2 and 3 go next.