Tektronix 577 Curve Tracer Adapter for Nuvistors & 7 Pin Triode/Pentode Tubes

The Tektronix 577 and 576 curve tracers are remarkable instruments from the 1970's & 80's, and rather affordable on the used market today. Designed primarily for characterizing semiconductor devices, these instruments can deliver 10s of amps, 100s of volts, step current and voltage, provide a wide range of loads while displaying currents and voltages in various formats and polarities.

I have had a Tek 577/D1 for many years, and thanks to Tektronix philosophy of the time of empowering owners with Right-To-Repair tools and manuals, it has been rather easy to keep it in nearly AS-NEW condition. But for me, discrete semiconductors are not the only thing these instruments can tackle. And Tektronix did offer a few more obscure adapters for integrated circuit and Op-Amp testing, but they are a rare find indeed these days!

But by stepping back 20 years before these instruments were introduced, we are in the era when vacuum tubes were king and the nascent semiconductor industry did not have such a clear future that would appear shortly. The electronics industry certainly saw the miniaturization and power-savings advantages that semiconductors offered, but cost, reliability, performance and availability were important design factors for new equipment.

In 1959 RCA introduced the Nuvistor tube as a miniaturized VHF-UHF capable all metal triode/tetrode device not much bigger than the discrete transistors of the late 1950's. Likely originally intended for television applications, they quickly found their way into many other consumer, commercial and military applications due to their reliability, size, performance and radiation hardness. I suspect that RCA engineers of the time foresaw these opportunities in spite of the advancing semiconductor business. By the mid 1960's nuvistors were everywhere high frequency, low noise, ruggedness and small size were ire important design factors.

I have a significant collection of nuvistor tubes, both NOS and used. But testing them with traditional tube testers of the day leaves something to be desired. My Superior Instruments TV-12 can't do it. And even my B&K 747, which can, just gives a single meter reading of relative trans-conductance. It may likely, that in the intervening 50 years since the introduction of the Tek 577, someone else has done what I describe in the following sections. But none the less, it makes for a rather interesting project I think.

This is not a treatises on curve tracer use. Others have that well in hand. But undeniably, the most common display of bipolar transistors on a curve tracer is the collector current versus voltage at various base current levels, generally called the common-emitter collector characteristics. They tell at a glance the input-output relationship of the device. Similar sweeps can be done for field effect devices, where gate voltage is stepped rather than base current.

But a field-effect semiconductor device is, in many ways, just a solid-state version of a vacuum tube (if we can disregard the pesky heaters)! With in reason, these modern Tek curve tracers have everything needed to display plate and grid characteristics with the proper fixture.

The above pictures shows one of the standard transistor test fixtures for the 576/577 curve tracers. The bottom pins are standard banana plug size. The three central pins are required for sensing and driving the device. I simply duplicated this adapter using nuvistor sockets as described in the next section.

The typical nuvistor is a three terminal device (if we can ignore the heater for a moment), so it easily conforms to the curve tracer drain, gate, source interface. The PCBs shown are machined in a CNC mill on single-sided copper boards using FlatCAM for isolation routing. The sockets are NOS nuvistor chassis mount types. The banana pins are threaded stud type and the binding posts provide terminals for filament power. Below are the gerber files.

The left photo above shows the plate characteristics for a used 6DS4 remote cut-off nuvistor, with a plate sweep of only to 25V. The step at low drain and grid voltages is due to the remote cutoff nature of the tube. The plate characteristics at higher voltages, as in the right photo, are typical of nuvistor triodes. The dual tube construction along with the Tek-177 adapter switch makes comparing device characteristics much easier.

I also built a very similar adapter for the minature 7 pin B7G tubes that are very commonly found in triode, tetrode and pentode configurations. For triodes and triode connected tetrodes & pentodes, the curve tracer is operated much like with the nuvistor adapter. For tetrode or pentode operation, (so long as the tube pinout conforms to 6AU6 type), the screen and suppressor grids can be jumper tied to either the plate or a bias supply directly or via a separate load resistor. As with the nuvistor adapter, the Tek 577 makes comparisons a lot easier.