Curve Tracers — Vol 2: A Short History

2.1 Tektronix invents the category

The curve tracer as a bench instrument is essentially a Tektronix invention, and it arrived in two waves — first for tubes, then for transistors.

The Tektronix 570 came first, introduced in 1955. It was Tek’s first curve tracer, and it was built for vacuum tubes: it swept the plate voltage from zero up to hundreds of volts while stepping the control grid through a set of bias voltages, and displayed plate, screen, or grid current against plate (or grid) voltage on its CRT. A front-panel pointer voltmeter monitored the DC and heater voltages, and it drew families of 4 to 12 steps. In other words, the 570 already had the full architecture described in Vol 1 — step generator, sweep supply, X-Y display — aimed at valves.

The Tektronix 575 followed in March 1957 and pointed the same idea at the newly important transistor. (The “575 = 1955” figure that circulates in some hobby writeups appears to conflate the 575 with the 570; the 570 is the 1955 machine, the 575 is the 1957 one — see the note at the end of this volume.) The 575 was produced from 1957 to 1972 and became the definitive first-generation transistor curve tracer. It used a gated Miller-integrator step generator to build the base-current staircase, and its collector sweep reached, in its original form, roughly 0–200 V at the 1 A range down to 0–20 V at the 20 A range; a later modification (Mod 122C) extended the high-voltage range to 400 V. If you have ever seen a photograph of a 1960s semiconductor lab, the 575 is the wrinkle-finish box with the round CRT and the rotating device sockets.

2.2 The classic bench instruments: 576 and 577

The Tektronix 576 (introduced 1969) is the machine most people picture when they say “curve tracer.” It kept the built-in 10 × 10 cm CRT with an internal parallax-free graticule but added an alphanumeric readout that printed the important front-panel scale factors right on the screen, so a photograph of the display was self-documenting. Its envelope was formidable — up to roughly 1500 V at low current on the high-voltage range, and up to about 20 A on the high-current range within a ~220 W dissipation limit. The 576 stayed in production into the following decade and remains a workshop favorite.

The Tektronix 577 (introduced November 1972, produced through 1993) was the lighter, plug-in-fixture companion to the 576. Instead of a fixed test station it used interchangeable standard and storage-display plug-ins and a family of test fixtures, which made it more flexible for production and teaching use. The 577’s long production life — into the early 1990s — tells you how durable the basic analog design was.

The line’s digital successors were the Tektronix 370 and 371, which arrived in the mid-1980s and replaced the analog CRT with a digital storage display and programmable operation (GPIB), while keeping the same fundamental step-and-sweep approach. Production of the classic Tek curve-tracer line eventually wound down, but 576s, 577s, and 370-series units remain the vintage iron people hunt for today (Vol 5).

2.3 The distinction that matters: curve tracer vs tube tester

It is worth pausing on a confusion that shows up constantly in the vintage-audio world: a tube tester is not a curve tracer. They answer different questions.

A tube tester — the emission testers and the better mutual-conductance (gm) testers like the Hickok and Heathkit machines covered elsewhere in this hub — applies a fixed set of bias voltages and reports essentially one number: emission current, or transconductance in micromhos (µmhos), at a single operating point. That is enough to sort “good / weak / short,” and gm testers give a genuinely useful prediction of how a tube will behave. But a tube’s characteristic is curved and nonlinear, and matching two tubes at one bias point does not guarantee they track across the range they actually work over.

A tube curve tracer — the 570, or a modern uTracer — draws the whole plate family (Vol 1’s tube figure). You can read plate current at any plate voltage for any grid bias, compare the curves to the datasheet, and match tubes across their whole operating surface rather than at a single point. The curve tracer is the strictly more informative instrument; the tube tester is faster and cheaper and, for go/no-go sorting, entirely adequate. This hub keeps both kinds of instrument precisely because they do different jobs.

2.4 Heathkit brings it to the hobby bench

Tektronix iron was expensive laboratory equipment. Heathkit put semiconductor curve tracing within reach of the hobbyist and the small service shop with the IT-1121 semiconductor curve tracer, introduced in 1973, and its later, electrically-identical restyle the IT-3121 (the two differ only cosmetically). These are add-on boxes: they generate the base-current staircase and the collector sweep and hand the X and Y signals to your own oscilloscope in X-Y mode rather than carrying a CRT of their own. The IT-1121/IT-3121 handled NPN, PNP, N- and P-channel devices, diodes (including Zeners and tunnel diodes), with collector voltage up to about ±200 V and current up to about 1 A across two ranges (roughly 40 V / 1 A and 200 V / 200 mA), from which you could read DC and AC beta, saturation voltage, breakdown, and linearity — and, importantly, match complementary pairs. It is a straightforward 1970s analog design (a couple of dozen transistors, a handful of 741 op-amps, one TTL counter driving the staircase), which is exactly why it remains repairable and hackable today. The Heathkit IT-3121 dive in this category goes deep on that machine and on modern modifications to it.

2.5 The modern DIY revival

For a couple of decades after the Tek line wound down, curve tracing meant either finding vintage iron or spending real money on a semiconductor parameter analyzer. Then the hobbyist community brought the instrument back, split along the same two-family line as the originals.

On the tube side, Ronald Dekker’s uTracer put a tube curve tracer on a small PCB by using a pulsed high-voltage technique (Vol 3): instead of dissipating hundreds of volts continuously, it charges a reservoir capacitor and applies the plate and screen voltages as brief pulses, measuring current during the pulse, so a tiny board can characterize a power tube without a bench full of regulated HV supplies. The lineage runs through the uTracer3 (now obsolete) to the current uTracer6 and the newer uTracer NXT, alongside the related eTracer — all covered in their own dives here.

On the semiconductor side, Paul Versteeg’s open-source VBA Curve Tracer (named for its three main contributors — Versteeg, Bud Bennett, and Mark Allie) revived the classic step-and-sweep semiconductor tracer as a modern, well-documented, build-it-yourself instrument that plots onto an ordinary oscilloscope in X-Y mode. It is explicitly aimed at being the thing you build when the school or shop’s aging 576/577 finally dies and cannot be repaired. The VBA Curve Tracer dive covers it in detail.

So the arc is tidy: Tektronix invented the category for tubes (570, 1955) and transistors (575, 1957), refined it into the classic 576/577 and then the digital 370/371; Heathkit democratized the semiconductor version for hobbyists (IT-1121/IT-3121, 1973); and the modern DIY community rebuilt both halves — pulsed-HV tube tracers and step-and-sweep semiconductor tracers — as open designs anyone can build. Vol 3 opens up how each of those actually works.

---

Date note (for the accuracy reviewer): Per TekWiki, the Tek 570 (vacuum-tube tracer) was introduced in 1955 and the Tek 575 (transistor tracer) in March 1957. This volume uses those verified dates. A “575 = 1955” claim appears in some secondary sources and in this subproject’s own scaffold notes; it appears to be a conflation of the two models and has been corrected here. The 576 (1969) and 577 (Nov 1972, through 1993) dates are from TekWiki; the 576’s exact end-of-production year varies by source (late-1970s to ~1990) and is left as “into the following decade.”