Curve Tracers — Vol 5: Buy vs Build

The reading skill from Vol 4 is portable across every tracer ever made; what changes is the box that draws the curves. This volume lays out today’s landscape — four routes to a working curve tracer — and points you at the specific instrument dives in this category by name.

5.1 Route 1 — vintage Tektronix iron (575 / 576 / 577 / 370)

What you get. A self-contained laboratory instrument with its own CRT, its own calibrated scale factors printed on the screen (576 and later), and a build quality that has already survived fifty years. The 576 is the sweet spot most people chase: the full-featured classic transistor lab tracer with a wide envelope (up to roughly 1500 V at low current, up to ~20 A at low voltage within its dissipation limit). The 577 is lighter and uses plug-in fixtures, which some prefer for teaching and production. The 575 is the earliest and cheapest but the most limited. The digital 370 / 371 add storage display and GPIB.

The catch. These are big, heavy, power-hungry, and now decades past their service life. Electrolytics, the CRT, range switches, and relays are all aging; documentation and calibration are on you; and a dead one can be an expensive doorstop. Prices have climbed as they became collectible. Buy vintage Tek iron if you want the genuine, no-compromise instrument and you are comfortable maintaining fifty-year-old analog gear — this hub’s whole restoration ethos lives here.

5.2 Route 2 — the Heathkit IT-1121 / IT-3121

What you get. A far smaller, simpler semiconductor curve tracer that hands its X and Y signals to your own oscilloscope in X-Y mode rather than carrying a CRT. Collector voltage to about ±200 V and current to about 1 A across two ranges (~40 V / 1 A and ~200 V / 200 mA), with a base-current staircase covering roughly 0.002–10 mA/step — enough to read DC and AC beta, saturation, breakdown, linearity, and to match complementary pairs. It is a straightforward 1970s analog design (a couple of dozen transistors, a handful of 741 op-amps, one TTL counter), which makes it genuinely repairable and modifiable on the bench. Typical working units traded around $200–$300 in the early 2020s.

The catch and the upgrade path. As-built it steps only negative gate voltage, so it traces JFETs but not enhancement MOSFETs (which need a positive gate offset), it produces only pulsating DC rather than true AC, and it tops out at 200 V. The active DIY community has answered all three: djerickson’s redesign around the IT-3121/IT-1121 adds a positive gate offset for MOSFET curves, swaps the multi-deck rotary switches for relay + microprocessor control, replaces the hardware polarity switches with dual 12-bit DACs for software-controlled positive/negative drive, and digitizes the display via ADCs — with an explicit goal of pushing the voltage envelope higher (“400 V good, 1 kV great”). The Heathkit IT-3121 dive in this category covers the stock instrument and these modifications in detail. This is the route if you want a real dedicated semiconductor tracer without vintage-Tek bulk, and you already own a scope.

5.3 Route 3 — the scope adapter and the octopus

Scope curve-tracer adapters occupy the middle ground: a small box (the Heathkit above is essentially one; there are others) that generates the step-and-sweep and uses your oscilloscope as the display. If you already have a good X-Y-capable scope, this is the cheapest way into real three-terminal curve tracing.

The octopus (Vol 3) is the cheapest route of all and the fastest to build — an isolation transformer, a series resistor, and two clip leads onto your scope’s X and Y inputs in X-Y mode. It will not give you β or gm families, but for in-circuit signature analysis — finding shorts, opens, leakage, and blown junctions on an unpowered board by comparing against a known-good reference — nothing is quicker or more forgiving. Almost every bench should have one; it complements, rather than replaces, a real tracer.

5.4 Route 4 — the modern DIY designs

The DIY revival split along the same two-family line as the originals (Vol 2). These are the sibling dives in this category, and each has its own deep dive.

For tubes — the pulsed-HV designs (Ronald Dekker’s line, plus the eTracer). These fit a several-hundred-volt tube characterizer onto a small board using the pulsed-HV technique from Vol 3, and plot the curves in host software on a PC.

• The uTracer6 is the current full-envelope Dekker generation. Per the designer’s specifications it covers roughly 0 to 1000 V at up to 1 A on the anode/screen supplies, with a 0 to −100 V grid supply (and an optional positive-grid capability) — enough to characterize serious sweep and power tubes. Its predecessor the uTracer3 is now obsolete.
• The uTracer NXT is the newer clean-sheet design; per the designer’s site it targets roughly 0 to 500 V at up to 350 mA on the anode/screen supplies with the same 0 to −100 V grid, in a more compact, community-enclosed package. (These NXT figures are as published on dos4ever.com and are worth confirming against the current build documentation before you rely on them.)
• The eTracer is the closely related pulsed-HV tube tracer covered in its own dive — the design in the same lineage that this hub owns and builds.

For semiconductors — the VBA Curve Tracer (Versteeg / Bennett / Allie). An open-source, thoroughly-documented step-and-sweep semiconductor tracer that plots onto an ordinary oscilloscope in X-Y mode via BNC X and Y outputs. It handles BJTs, JFETs, MOSFETs, and Darlingtons (two- and three-terminal parts), with three selectable ranges — roughly 0–35 V @ 2 A, 0–70 V @ 1 A, and 0–200 V @ 100 mA — and offers both stepped-current and stepped-voltage drive so it can do the MOSFET job the stock Heathkit cannot. It is explicitly designed as the instrument you build when a lab’s or school’s aging 576/577 finally dies and cannot be repaired. The VBA Curve Tracer dive covers the build.

5.5 Choosing — a short decision guide

Match the tool to the device family first:

Table 1 — Match the tool to the device family first:

If you mainly test…	Reach for	Why
Vacuum tubes (matching, characterization)	the uTracer6 / the uTracer NXT / the eTracer (DIY, pulsed-HV)	Small, safe pulsing, host-side plots, full plate families a tube tester cannot give
Vacuum tubes, and you want the original iron	Tektronix 570	The genuine article, if you can find and maintain one
Transistors / diodes / FETs, dedicated instrument	the Heathkit IT-3121 or the VBA Curve Tracer	Real step-and-sweep families onto your scope; VBA adds MOSFET/positive-offset drive
Transistors, no-compromise lab instrument	Tektronix 576 / 577 / 370	Self-contained, wide envelope, calibrated — at the cost of size and upkeep
Dead-board fault finding, in-circuit	An octopus V/I tester	Fast signature analysis, unpowered, no desoldering

And the buy-vs-build question itself: buy vintage iron if you value the finished, calibrated instrument and enjoy restoration; build a DIY design if you want a modern, documented, repairable instrument you understand top to bottom — and, for tubes especially, if you want the compactness and pulse-safety that only the modern pulsed-HV designs offer. This hub deliberately does both: it keeps and restores vintage gear and builds the modern DIY tracers, because the DIY-vs-buy duality runs through every instrument class on this bench.

Whichever route you take, the reading skill is the same one from Vol 4 — the box just changes. Follow the cross-links into the eTracer, the uTracer6, the uTracer NXT, the Heathkit IT-3121, and the VBA Curve Tracer dives for the instrument-specific detail this primer intentionally leaves to them.