Most tube curve tracers published on the web consist of a set of programmable voltage sources - say two 0 to 400 V sources for the anode and screen grid, and a 0 to -50 V source for the control grid – which are connected via some matrix switch board to the tube to be tested. After the proper voltages have been applied to the electrodes, the anode and screen currents are measured and plotted, and the next bias point is set. The problem with this method is that since the actual current measurements only takes a very small amount of time, an awful amount of energy is wasted both by dissipation in the tube itself, as well as in the power supplies.
The basic idea behind the uTracer is that in order to measure the anode and screen currents, it really isn’t necessary to have the tube switched on all the time. With this I obviously don’t mean the heater, that one has to be on at least a minute or so to stabilize. What I refer to are the high voltages, they only need to be applied to the tube a fraction of a second before the currents are measurement, and they can be switched off immediately afterwards!
Overview of the circuit diagram of the uTracer tube tester / curve-tracer.
During such a pulsed measurement, the high-voltage power supplies only have to deliver power (P) for a very short time, while the total amount of energy (P*t) remains low. This simple notion greatly simplifies the design of the power supply. Remember how the circuit of an electronic camera flash-light works? A tiny circuit charges a large reservoir capacitor until a certain voltage is reached. During the flash, the reservoir capacitor delivers in a fraction of a second hundreds of Watts to the Xenon flash-tube, but since the time is so short, the total amount of energy is still quite low. The power supplies in the uTracer basically work the same way. A tiny boost converter, in reality nothing more than an inductor, a transistor and a diode, charge a large electrolytic reservoir capacitor to the required voltage. Then, only during a millisecond, the charged capacitor is connected to the tube via an electronic switch. During this millisecond the currents are measured. Immediately after the measurement, the capacitor is again disconnected from the tube. During the measurement the reservoir capacitors are discharged to some extent. This is measured, and taken into account for. After the measurement cycle, the boost converters charge the capacitors to the next bias point so that they are ready for the next measurement.
The whole process is controlled by a fast micro-controller, which performs all the necessary tasks simultaneously. First of all it takes care of the charging (and sometimes also discharging) of the reservoir capacitors. Secondly, it controls the pulsed measurement itself: programs the current amplifiers, measures the currents and communicates the results to the Graphical User Interface (GUI) on the PC. Next to that there are a few other tasks running on the back-ground such as: controlling the inverting boost-converter for the negative power supply, and controlling the heater and control-grid voltages. The uTracer has a “hard-wired” over-current protection which limits the maximum currents to 220 mA. Next to that there is an interrupt driven programmable over- current protection which switches off the high voltage supplies in case a full short circuit situation occurs. Through the efficient combination of hard- and software it was possible to reduce the hardware for the uTracer to the absolute minimum.
Only an old laptop power chord and a few external components are required to turn the uTracer into a tube curve-tracer.
The uTracer uses an (old) laptop power chord (17 – 22 V) as a power supply. Since the heater of a tube is basically a resistive load, a simple Pulse Width Modulation circuit could be used to adjust the heater voltage between zero and the power supply voltage. These power chords usually can deliver 50 Watt or more, so that even relatively high heater powers such as for the popular EL34 (6.3V / 1.5A) can be generated without a problem.
In the next section the complete circuit diagram of the uTracer tube tester / tube curve-tracer is discussed in greater detail.