A Microprocessor Controlled Valve Tester
G8LSD's completed board. Click for large image.
The μTracer (search for utracer on Google) is a single board microprocessor controlled valve characteristic measuring tool. The μTracer has been designed by Ronald Dekker and kits are available from his website.
Profile of the board and connectors for power (left) and heaters (right).
All voltages, except the heater, are only generated for a milisecond or so - just long enough for a reading to be taken. In this way the valve under test is not driven too hard and lethal voltages are not permanently present.
Block diagram of the μTracer from Ronald's website.
Construction of G8LSD's μTracer
A small section of the board. Note the clear legend.
The kit comes with a detailed and comprehensive assembly manual. Construction is divided into ten stages: each is a case of build and then test before moving on. The professionally made board has very clear lettering and polarised components are clearly marked as to orientation. The fibre-glass board measures 162 x 100 mm and has wiring printed on both sides with plated through holes where appropriate. With a fine pointed soldering iron bit and 22 gauge cored solder the components are easy to assemble.
A resistor prepared for insertion.
I was taught, in the days of carbon resistors, to form component leads in a way that did not put strain on the ends of devices. The image above illustrates the method. The lead is held very close to the body of the component with the tip of the wireman's pliers and then the lead is bent over the tip of the pliers and almost back parallel with the body of, in this case, the resistor to form the first curve. The pliers are then inserted into the loop and the lead bent outwards again. In this way the leads are made as wide apart as required and they stand off the board for good air circulation.
Wiremans pliers - obtained second hand in 1968. They are 130mm long and 46mm across the handles.
Operation of the μTracer is via a Graphical User Interface (GUI) running on a PC. This communicates with the microprossor on the board via an RS232C serial interface. Modern PC's and laptops without a serial port will need a USB to RS232C converter. I purchased mine from Farnell (UK supplier).
USB to serial board. This one has the correct chip set.
The 3 mm fibreglass front panel. Connectors fitted are for 20 Volts DC input power and the USB link to the PC.
The inside of the galvanised steel chassis with USB module fitted.
Main board on aluminium angle with spacers.
The spacer has a stud one side and a thread the other.
The main board fitted. All aluminium brackets are held in place with two-pack apoxy adhesive.
Fitted main board side view.
The front of the USB board rests in the front panel.
Final stages of construction of the main unit. The veroboard unit contains the input fuse and choke and the heater fuse and choke. Eight of the nine way terminal block connections are used. Ribbon wire connects to the main board. The connectors on the front panel are 2mm sockets.
The completed front panel in place within the cabinet.
The completed main unit.
The valve holders are mounted in plastic boxes and held in place on the steel cabinet with magnetic strips.
The GUI almost fills a screen 1024 x 768 pixels and sub screens include a comprehensive calibration page that is saved to disc and ensures that voltages produced by the μTracer agree with the builder's own multi-meter. The calibration screen has a series of sliders that allow very precise control of the μTracer output.
Data produced by the tracer is shown as a graph and the plot values can be saved to disc.
In September 2014 I moved the calibration file to the Museum three screen HP computer and began to measure some valves. To be able to cover a wide range of valves, including some power valves, I purchased a cheap power supply to act as an external heater supply.
Once a particular valve Type has been correctly set-up it is simple to save the set-up for repeat use. Over time a library of types is therefore compiled. The plan is to measure some of the older valves for which we have no data-sheet and to select audio valves for demonstration projects.
Having made leads to connect between the valve image on the tester and the nine sockets on the valve holder carrier it became quickly apparent that this was a very successful way of setting up for a new valve. The electrode ends of the leads are colour coded and the other ends are all blue. The heater leads from the external power supply have red and black plugs to make recognition easy.
A plot of a good ECC83.
For plotting double triodes like the ECC83 above one anode connects to the anode supply and the other anode to the screen supply. The tracer then plots both sections at the same time. The plot above shows a closely matched pair of triodes. Sadly most that have been measured have poor in comparison.
The screen display for the above graph.
A good general test to check the tracer itself is to plot a medium power valve such as the EL84, 6BW6 or KT61.
A set of curves for the KT61.
As the μtracer can supply 200 mA from the HT supplies the KT66 can also be measured well within the units capabilities.
A set of curves for the KT66.
A useful feature is the Quick Test facility. This performs a set of measurements at a single bias point and after statistically verifying the set will evaluate the basic valve parameters of Ra, gm and μ together with a scale of anode current at the bias point v makers expected value. Such a test for the KT66 is given below.
KT66 quick test window.