Introduction

I have been using my uTracer 3+ for approximately three years and, during that time, I have tested thousands of tubes. After using several classic tube testers, I can confidently say that, for me, this is the most versatile and fastest tester I have ever used.

The uTracer 3+ is not simply a device that reports whether a tube is “good” or “bad”. It is a genuine curve tracer for thermionic tubes: it allows the user to observe the electrical behaviour of a tube at different operating points, measure important parameters and compare the results with the conditions specified in tube data sheets.

The major difference compared with many older testers is that the user is not limited to an emission scale or a single transconductance reading. With the uTracer, a tube can be analysed in a much more complete way.

What is the uTracer 3+?

The uTracer is a project developed by Ronald Dekker and originally made available as an electronic kit. The 3+ version is a direct evolution of the uTracer 3 and extends the high-voltage range to 400 V. It uses a GUI application on a computer and communicates through a serial interface.

The kit provides the electronic core of the tester. The enclosure, sockets, switching matrix and many accessories depend on the way each user builds the instrument. This may initially appear to be a limitation, but it is also one of the project’s greatest strengths: the system can be adapted to the tube types most frequently tested by each user.

In my unit, this flexibility is essential. I can work with small-signal tubes, power tubes, dual triodes, pentodes and different socket types without being restricted to the fixed configuration of a traditional tester.

The uTracer 3+ is designed to work with GUI 3p11 or later, with the “Version 3+ (400 V)” hardware option correctly selected in the calibration file.

How it works

The uTracer operates differently from a traditional instrument based on large, continuously operating high-voltage power supplies.

The board contains boost converters that charge reservoir capacitors to the programmed voltages. During a measurement, the tube receives a very short pulse, normally around 1 ms, and the microcontroller measures the corresponding currents. The voltage is then disconnected and the capacitors are charged again for the next measurement point.

This method provides several advantages:

It reduces the energy dissipated during testing.
It allows the use of a relatively small external power supply.
It makes fast curve tracing possible.
It reduces the risk of destroying a tube through excessive stored energy.
It allows brief testing at operating points that would not be suitable for continuous operation.

The last point is important. If a curve briefly exceeds the maximum dissipation stated in a data sheet, this does not mean that the tube can operate continuously under those conditions. The pulse is very short and must not be confused with a permanent thermal operating condition.

What can be measured?

The uTracer 3+ can configure and measure, among other parameters:

Plate voltage, Va.
Screen voltage, Vs.
Control-grid voltage, Vg.
Plate current, Ia.
Screen current, Is.
Output curves.
Transfer curves.
Transconductance, gm.
Internal or plate resistance, rp.
Amplification factor, μ.
The behaviour of dual triodes.
Comparisons between tubes.
Load lines and dissipation limits.

For a triode, these parameters are related by:

[ \mu = g_m \times r_p ]

In practical terms, this makes it possible to move beyond a purely qualitative evaluation and analyse the way a tube is likely to behave in a real circuit.

For example, the two sections of an ECC83 can be tested separately, the transconductance of different EL84 tubes can be compared, or tubes with sufficiently similar characteristics can be selected as a matched pair.

Quick Test: speed for everyday work

Although full curve tracing is the most impressive feature, Quick Test is probably the most useful function for anyone testing large numbers of tubes.

After the tube type and operating point have been selected, the uTracer applies the chosen conditions and quickly displays the main measured values. For a triode, the most relevant parameters are normally:

Plate current.
Transconductance.
Plate resistance.
Amplification factor.

For screening, grading and cataloguing, this mode is much faster than performing a complete curve trace for every tube. In my case, this is one of the main reasons why the uTracer has become my most frequently used tester.

Speed, however, should not lead to rushed measurements. The heater must be allowed to warm up and the tube should be given time to stabilise. A measurement taken too soon may primarily reflect the heater’s transient behaviour rather than the tube’s normal operating condition.

Accuracy: what can be expected?

The uTracer 3+ is very accurate for its intended purpose, provided that it is correctly calibrated and installed. I would not classify it as a laboratory metrology instrument, but for servicing, circuit development, tube selection, radio and amplifier repair, and vintage-component cataloguing, the results are extremely useful.

Three points should be distinguished:

Electronic accuracy. This depends on calibration of the voltage converters, current amplifiers and grid-bias circuit.
Repeatability. This is normally very good when the heater conditions, wiring and configuration remain unchanged.
Comparison with data sheets. The exact Va, Vs, Vg and heater conditions specified by the manufacturer must be used.

When comparing EL84 curves with the corresponding data sheet, a very good agreement was observed, but it is clear that no metrological calibration against a reference instrument was carried out.

It is also necessary to remember that tubes themselves vary. Two new tubes of the same type and manufacturer may not produce identical readings. Depending on the manufacturer, production batch, test conditions and manufacturing tolerances, differences of approximately 10–20% may be entirely normal.

For this reason, I consider it more important to assess the complete set of results than to focus on a single number:

Does the curve have a normal shape?
Does the tube show leakage or instability?
Are the two sections of a dual triode reasonably similar?
Is the transconductance consistent with the measured current?
Is the result repeatable?
Does the tube respond normally to changes in grid voltage?

The importance of calibration

Calibration is a fundamental part of the system. The uTracer measures voltages and currents through its own reference circuits, so the calibration file must correspond to the board, hardware configuration and correct GUI version.

The following areas should be checked, among others:

Supply voltage.
Gain of the plate and screen supplies.
Gain of the current channels.
Grid bias.
Grid accuracy near 0 V.

The construction manual recommends a staged calibration procedure and includes tests using known resistors to verify the current channels. Grid calibration near 0 V is particularly important because small errors in this region can have a significant effect on tube current.

For a user who tests thousands of tubes, a maintenance record is good practice. It should include:

Date of the last calibration.
Power supply used.
Serial adapter used.
Approximate laboratory temperature.
Reference tube or resistor.
Repeatability test results.
Changes to wiring or sockets.

Reliability after thousands of tests

My experience, after approximately three years and thousands of tested tubes, has been very positive. The instrument remains fast, stable and versatile, without losing its usefulness under intensive use.

This experience is consistent with information published by the project’s author. The official website has reported more than 1,800 working uTracer 3 units and described the project as robust against incorrect operation, short circuits and other incidents. Later information mentions more than 1,900 units reported as working.

These figures should be interpreted carefully: they are not an independent reliability certification or a statistical failure rate. Nevertheless, they are a relevant indication of the project’s maturity, particularly because the uTracer is a user-assembled kit rather than an industrial instrument manufactured on a fully automated production line.

Part of its robustness comes from the pulsed measurement method. The energy available in the reservoir capacitors is limited, reducing the likelihood that an oscillation or short circuit will immediately destroy the tube. However, the board is not indestructible. A prolonged short circuit or incorrect connection can damage high-voltage switching components and control circuits.

The most common problems

1. GUI installation problems

According to the official documentation, most reported difficulties are related to GUI installation or operation rather than to the board’s electronics.

The original GUI is an older Windows application based on Visual Basic components and OCX files. On modern versions of Windows, users may encounter:

Missing OCX files.
MSCOMM32.OCX registration errors.
ComDlg32.OCX errors.
“Run-time error 52”.
Permission problems in Program Files.
Installation problems on 64-bit versions of Windows.

The usual solution is to use the correct GUI version, install the 32-bit components and, when necessary, register the OCX files in the SysWOW64 folder with administrator privileges. The safest alternative is to keep a dedicated Windows computer or virtual machine for the uTracer.

2. Incompatible USB-to-serial adapters

The uTracer communicates through a conventional serial interface. The manual recommends a USB-to-serial adapter using a good-quality FTDI chipset, configured for 9600 baud, one stop bit and no parity.

Poor-quality adapters can cause:

Communication failures.
Timeout errors.
Interrupted readings.
Loss of connection during a curve trace.
Apparently random behaviour.

In my opinion, saving a few euros on the adapter is false economy. A good FTDI adapter eliminates one of the most common sources of trouble.

3. Wiring that is too long

Tube-socket wiring is critical. Long wires, matrices with many switches and parasitic capacitance can produce oscillations or irregular curves. This problem may be more noticeable with small-signal tubes than with power tubes.

Users have reported that a connection working correctly with a socket mounted close to the board can become unstable when routed through an extensive wiring and switching matrix.

To reduce this risk:

Keep connections as short as possible.
Use tidy, symmetrical wiring.
Keep socket wiring lengths similar.
Physically separate high-voltage conductors.
Use ferrites or RF-suppression beads where appropriate.
Avoid leaving unused sockets and wires connected to the circuit.

If a curve appears jagged, unstable or oscillatory, the first step should be to test a single socket directly beside the board. Only then should the software or tube be investigated.

4. Heater-circuit problems

The internal heater supply uses PWM. A normal multimeter may not accurately represent the effective voltage applied to the heater.

The PWM waveform and parasitic inductance in the wiring can result in a lower effective heater voltage than expected, especially at lower heater settings. For this reason, many users prefer an external heater supply when maximum accuracy is required or when testing particularly sensitive tubes.

An external supply is also useful for:

High-current heaters.
Directly heated tubes.
Tests requiring AC heater power.
Battery tubes.
Comparisons requiring the exact conditions specified in a data sheet.

5. Interrupting the discharge period

A new measurement should not be started, and buttons should not be pressed repeatedly, while the high-voltage reservoir capacitors are discharging. The documentation states that this can leave the software apparently locked because the firmware is not accepting commands during that period.

The rule is simple: after interrupting a test, wait until the high-voltage LED goes out before starting another operation.

6. Jumpers and assembly issues

Erratic behaviour during assembly may be caused by:

A PIC inserted incorrectly.
A missing J3 jumper, leaving the reset unstable.
Incorrect J2 configuration.
An incorrect crystal frequency.
An oscillating 5 V regulator.
An unsuitable 19 V power supply.
Reversed power-supply polarity.

The manual recommends following the assembly sequence carefully and testing each circuit block before proceeding.

Limitations to understand

The uTracer 3+ is excellent, but it is not universal.

The maximum plate and screen voltage is approximately 400 V, and normal current is limited to around 200 mA. This covers the great majority of radio, television and audio tubes, but not every high-power application.

It is not the ideal instrument for:

Continuous high-dissipation tests.
Tubes requiring more than 400 V under normal conditions.
Currents significantly above 200 mA.
Full positive-grid-current measurement without modifications.
Direct Class A2 operation without the appropriate extension.
Industrial testing with certified metrological traceability.

For applications requiring 1,000 V, 1 A or an extended grid-bias range, the uTracer 6 is more appropriate, although more complex and more sensitive to oscillation problems. The project’s author considers the uTracer 3+ suitable for approximately 90% of common tube types.

Comparison with classic tube testers

Feature	Classic tube testers	uTracer 3+
Typical result	Scale reading or single value	Complete curves and parameters
Speed	Highly variable	Very fast in Quick Test mode
Flexibility	Limited by internal circuits and sockets	Highly configurable through software
Transconductance	Available on some models	Measured under defined conditions
Screen current	Not always measured	Can be measured directly
Data-sheet comparison	Usually indirect	Directly through plotted curves
Tube matching	Good for screening	Excellent for detailed comparison
Repair	May require specialist knowledge	Repairable by an electronics technician
Interface	Analogue	Computer and serial interface
Maintenance	Ageing components and mechanical calibration	GUI, calibration and wiring
High voltage	Often continuous	Pulsed and energy-limited
Initial ease of use	Usually immediate	Requires assembly and configuration

Classic testers still have value. They are fast for certain checks, offer a distinctive user experience and may be the best tool for particular historical procedures. However, in versatility and the amount of information obtained from each test, the uTracer is clearly in another league.

My practical experience

What I appreciate most about the uTracer 3+ is the ability to adapt the test to the objective.

If I am screening tubes quickly, I use Quick Test. If I want to study a tube, I trace the complete curves. If I want to compare both sections of an ECC82, I perform exactly the same test on each section. If I want to select tubes for sale or for an amplifier, I save the results and compare not only current, but also transconductance and curve shape.

After thousands of measurements, its greatest advantage has become clear: the instrument is no longer just a tester; it is also a documentation tool.

Each tube can be associated with:

Serial number or internal reference.
Manufacturer and branding.
Tube type and construction.
Test date.
Heater voltage.
Operating conditions.
Plate current.
Screen current.
Transconductance.
Curve graphs.
Visual and electrical observations.

For anyone working with vintage tubes, this information is much more valuable than a simple “passed” or “failed” label.

Recommended procedure for consistent results

Confirm the tube pinout and socket type.
Check the heater voltage and current rating.
Visually inspect the tube for damage, getter condition, base problems and dirty pins.
Confirm the calibration and correct GUI file.
Switch on the heater and allow it to stabilise.
Use the exact data-sheet conditions when comparison is required.
Start with a conservative compliance-current limit.
Perform a low-stress Quick Test first.
Trace complete curves only after confirming that the tube responds normally.
Save the configuration file and results.
Repeat the measurement if instability or unexpected values appear.
Confirm suspicious results using a second method or tester.

With directly heated tubes and rectifiers, particular attention must be paid to the connections between heater, cathode and any external supply. An incorrect connection can produce false results or cause damage.

Safety

Although it is powered by a low-voltage external supply, the uTracer generates high voltages internally. The reservoir capacitors may retain a dangerous charge.

The board should never be handled merely because the high-voltage LED is off. The construction manual expressly warns that the indicator must not be treated as the only safety check; the voltage should be confirmed with a multimeter.

Essential rules include:

Use a closed enclosure.
Install suitable fuses.
Disconnect power before changing connections.
Wait for the capacitors to discharge.
Confirm the voltage with a multimeter.
Use insulated cables and connectors.
Do not leave high-voltage terminals exposed.
Use one-hand working practice whenever a hazardous measurement is necessary.
Keep children and unauthorised people away from the equipment.

Conclusion

After approximately three years of use and thousands of tested tubes, my verdict is unequivocal: the uTracer 3+ is one of the most complete, fast and versatile testers available to anyone working with thermionic tubes.

Its greatest strength lies in the combination of complete curves, speed, flexibility and relatively low cost. It does not replace every classic tube tester and is not intended to be an industrial metrology instrument. It also requires more knowledge, calibration and care than a commercial bench instrument.

However, when properly built, calibrated and wired, it provides an extraordinary amount of information. It allows tubes to be tested, compared, selected and documented in far greater detail than many older testers can provide.

The experience of other users confirms the project’s maturity: the electronics are generally regarded as robust, while the most common problems involve the older GUI, serial adapters, wiring, parasitic oscillations and the PWM heater supply.

For my work with vintage tubes, the conclusion is simple: classic testers still have their place, but the uTracer 3+ has become my reference tool. It is fast when speed is needed, detailed when information is needed and flexible enough to support practically any tube-testing project I want to develop.

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uTracer 3+ Review: Accuracy, Reliability and Practical Tube Testing