Old dog, newer trick

Although mostly obsolete, the vacuum tube still has its adherents among audiophiles, test-instrument collectors, amateur-radio fans, and those curious about the physics of active devices. Given that a family of characteristic curves is worth a thousand data points, being able to view a device’s behavior enhances a small laboratory’s capability.

Vacuum-tube curve tracers are expensive. Demand by audiophiles has pushed asking prices for Tektronix’s 1956-vintage Model 570 well over $1000, making this rare instrument unaffordable for casual experimenters. An alternative, the Model 575 transistor curve tracer, typically sells for under $400 but requires accessories to display vacuum-tube characteristics.

A recent series of messages in Yahoo’s TekScopes forum inspired me to explore what’s needed to plot tube curves within the limitations imposed by the Model 575’s transistor-specific capabilities. These include a lack of tube sockets, the absence of heater (or filament) and screen-grid power supplies, limited base (or grid) voltage-step size, and a 200-V maximum collector (or plate) voltage supply. Tektronix addressed the latter shortcoming via its factory-upgraded Type 575 Model 122C and also offered the 122C upgrade kit, which is likely unobtainable nowadays. For testing many small-signal tubes, most users can live with the 575’s 200-V limit. An external amplifier module can boost the grid-voltage steps by a factor of 10, and an inexpensive multivoltage switched-mode universal power supply can warm a tube’s heater.

Traditional tube testers solve the tube-socket problem by providing one of every commonly used socket type and a matrix of switches to connect a DUT’s (device under test’s) socket pins to the tester’s circuitry. Since my requirements involve either an extensive exploration of a single tube’s characteristics or the testing of a batch of identical tubes, I opted for a simpler approach (see photo). A small printed-circuit board provides an interface between the 575, external power supplies, and a 12-pin connector for the tube socket. Two plugs and jumper wires connect the DUT’s pins to the curve tracer, enabling my old Model 575 to plot vacuum-tube curves—a newer trick for an older dog.

Making Connections
To bridge the gap between the 575 and a vacuum-tube DUT that requires from four to 12 connections, the interface panel provides connectors for external filament (or heater) and screen-grid power supplies. Two groups of flying leads connect to the 575’s emitter, base, and collector binding posts. The DUT socket mounts separately and connects to the interface panel. If needed, ferrite beads on the DUT’s grid and plate wires help suppress parasitic oscillations. The remaining sockets accommodate a pair of plugs serving as a removable “patch panel” for configuring the DUT socket’s pinout to the interface.A small universal power supply intended for digital cameras delivers a switch-selectable DC output of from 3 to 7 V at up to 2.1 A to the DUT’s filament or heater. If required, an adjustable regulated DC power supply provides screen voltage.

Boost the Output
A 575’s base/grid step generator’s limited output benefits from an external amplifier. This Linear Technology application note describes a suitable high-voltage amplifier featuring either transistors or a vacuum-tube output stage: cds.linear.com/docs/Application%20Note/an18f.pdf

Using a Tek 576 or 577
Daniel Schoo offers an introduction to tube curve tracing and shows how to use these later-model semiconductor curve tracers to display tube curves: bit.ly/w2hroZ

Reference book
This book provides a definitive introduction to curve tracers, tube testers, and other vintage instruments:
Douglas, Alan, Tube Testers and Classic Electronic Test Gear, Sonoran Publishing, 2000. 166 pages.