Construction and performance of the popular µTracer vacuum tube analyzer kit. Also covered is the addition of a precision, switching, heater regulator for better accuracy. I call the combo, the µTracer/HR.
Part I - Overview, Heater Regulator, Look Inside
Part II - µT in Action, Accuracy, Cost, Docs
Part III - Gallery, Vinyl Wrap, Parts, Building HRB
Part IV - Chassis Assembly, System Test
Ronald Dekker’s µTracer 3+ partial kit at left has been available since 2012 and has since become widely used in the hobbyist community. With some 1400 units shipped, it is, without a doubt, a run-away best seller among kits of this nature. The winning combination of good performance and affordable construction makes it quite unusual among available tube testers. This small but powerful tube tester and curve tracer can be purchased as a partial kit for €225 (presently $253) including shipping. You just provide an enclosure, pin switching, a laptop-type AC adapter and a few other parts. Units have been shipped to some 51 countries from the Netherlands. The µTracer makes pulsed measurements lasting only ~1ms, whereas conventional analog testers make continuous-time ones. The µTracer also requires a Windows PC (left) which provides the user interface and graphics output.
Although I had already built the Vacuum Tube Analyzer (VTA) covered in another article, I decided to buy a µTracer kit and build this project around it. Why? I want it for plotting tube characteristics automatically. Those can be plotted manually using the VTA but it’s far easier with the µTracer and therefore more likely to happen. I also wanted to have another instrument for comparison with the VTA results.
Seen at left and at the top of the page, four tube sockets support 7-pin mini, 9-pin mini, octal and compactron tube bases, covering a large swath of receiving types, including the Sovtek 6L6WXT shown. The compactron socket also serves as an expansion connector for accessory boxes to support other bases. Extra room and faint markings on the top panel also make it easy to add up to four more tube sockets, if desired.
The panels are covered with a tough, vinyl car wrap, providing attractive graphics and labeling, as well as an accurate, informative template for drilling. By the way, the bottom cover will be painted “Tektronix blue,” matching the blue lettering.
On the sloping front panel we have a row of 2-pole (DC-type) jacks representing tube pins. The red row of banana jacks also connects to those pins, giving convenient access for test equipment. The third row has the voltage sources, labeled in blue, plus a heater voltage selector switch (5.0, 6.3, 12.6V) on the left and a power switch with indicator on the right. Also on that row, labeled in gray to inhibit casual use, is the heater source from the main board, which should be used judiciously. (See below.)
Also on the front panel is the red high voltage warning light and a ground connection for test equipment. An array of six, unconnected, 2-pole jacks insures that unused patch cord connectors can always be kept safely with the unit, so they won’t be lost. The rear panel seen in the lower picture has the power cord, computer interface connector and DC fuse. Fusing for the AC line is included in the internal AC adapter.
Adding a Heater Regulator
Please note that, while I will be direct about discussing limitations of the µTracer, I have great respect for Dekker’s excellent creation. The cost-effectiveness that he has achieved is nothing short of amazing! It has brought a new level of tube testing accuracy to a huge number of hobbyists and I am a very satisfied customer. The tender loving care which went into every aspect of the kit, from the meticulous assembly instructions to the carefully bagged and labeled parts is heart-warming. Kudos to Ronald and his wife, Marie-José, for all of their devotion to this labor of love. They continue to provide and support the kits—I periodically receive emails discussing new uses for the µTracer and ways to extend its functions. For example, a recent one discusses small modifications you can make to extend its plate current range to 600mA and more! [Why would you want more than the original 200mA limit? For testing power tubes like the 6550, as discussed here.]
The Issue with the Heater Supply
There is a discussion in the VTA article which compares it to the µTracer, identifying certain limitations. I will only mention the main one here: heater voltage accuracy (but I invite you to click the link for more). Dekker has written about the heater voltage issue on his extensive website. The µTracer provides variable heater voltage by generating a 20kHz pulse-width-modulated (PWM) signal of 19V from the main DC supply. These unfiltered pulses are applied to the tube’s heater. The voltage across the heater at left spends most of its time at zero. There is no problem with interference from this because the switching is cleverly stopped during the short measurement interval. The problem is simply that the effective heater voltage is affected by the limited time resolution of the PWM and by parasitics which change the waveform. Ferrite beads are needed in the tube socket wiring to avoid VHF tube oscillations but the inductance from the beads can adversely affect the heater waveform and hence, the effective heater voltage. See Appendix A for more on how unfiltered heater pulses affect accuracy.
The issue is complicated by the fact that it’s difficult to measure the effective heater voltage accurately. Only a true RMS (TRMS) measurement can give us the right value. An ordinary, averaging meter will not show the effective heating value of the pulsed waveform. But the dirty little secret of most TRMS meters is that they lose accuracy for signals like this, with high peak-to-average ratios. Moreover, accuracy is reduced due to the wide bandwidth of the narrow pulses. With the heater voltage set at 6.3V, my two best TRMS meters (at left, Tektronix 4050 top and HP3455A bottom) showed 6.017 and 5.953Vrms, disagreeing about 1.1%. They agree to 0.2% with a 1kHz sine wave. (Note that the pics were taken later with a sine source to give a representative display.) Averaging the two original values gives 5.985V, which is 5% below the target. One could try to find a setting which will measure 6.3Vrms and use that but with the best available meters disagreeing by over a percent, I wasn’t confident in the measurement.
A Precision, Switching, Heater Supply
The solution was to build the precision, switching regulator board at right to provide selectable DC heater voltages at up to 3A continuously (2A at 12.6V). (Handles higher currents for a limited time.) The three voltages available (5.0, 6.3, 12.6) cover most receiving tubes and any others can be accommodated by using an external lab supply. It uses a switching regulator module available on eBay for just $2 each in a pack of five, including shipping. The rest of the circuitry wraps a precision, remote-sensing regulator (with a 1ppm/C voltage reference) around the module and provides extra LC input and output filtering to suppress switcher noise. The 0.1%, low-tempco divider resistors support a narrow, ħ1% adjustment range. That, along with the precision opamp and voltage reference, insures drift is held to just 0.1% or less. It typically holds the output within a couple millivolts, at the socket. The goal was to have it so solid and accurate that there is no need to check it in normal use. See Appendix B for a discussion of the heater regulator design. Full documentation and a source for the PCB are at the end of Part II.
Inside the µTracer/HR
Above, all of the electronics are mounted to the top section of the chassis. The bottom cover is just a cover. In this view, the µTracer main board sits above the tube sockets on 1.5 inch standoffs. At left we see the rear panel items, including the DC terminal strip with the inductor and the AC terminal strip below with the mains connections. (There will be a clear plastic shield above the AC connections but I still need to locate a thin, clear, plastic, L-shaped piece for that.) The 65W AC adapter is mounted at the corner of the rear and top panels using contact cement. This gives it maximal thermal contact with the aluminum panels for heat dissipation. Contact cement is expected to provide a robust mount (it’s used to attach laminates to countertops) but since it stays pliable, the AC adapter can be pried loose if it fails.
To the right, the switching heater power supply is mounted to the back of the front panel where there is plenty of aluminum area to help dissipate heat. With the red switching regulator module being a second level above the PCB and the front panel sloping down close to the bottom cover, I had to be careful to insure there is adequate clearance.
Below, the main board is flipped-up into service position, revealing the socket field wiring, loaded with oscillation-suppressing beads. A key design goal is to make all parts of the unit easily serviceable. To support that, all wires going to the PCB must attach from one side. The heater regulator board can flip up like this too.
Below is a close-up view of the socket field wiring, with all the ferrite beads. Most wires going to a tube pin must have a bead. The exceptions are short wires linking the left two sockets and separately, wires linking the right two sockets. Following Dekker’s suggestions, pin-1 (for example) is routed from the connector to one end of the socket field (through a bead) and looped through the sockets (placing a bead in the middle) and then returned to the connector, through a bead. A separate sense-wire runs from the second connector pole to the reference socket (usually the octal), through a bead.
Coming up next: µTracer in action, accuracy tests, cost and documentation...
Copyright © 2019, Stephen H. Lafferty
January 29, 2023 - 08:34 am|
|Hi Paolo, No question is trivial if it matters to a Tronola reader! The part of the article dealing with the PCB is here: |
Getting the Heater Regulator PCB
As mentioned there, Elecrow looked like a good vendor when I wrote the article. In the second paragraph of that section, please click the link, "PCB data files are here" to get the files you need to send to the vendor. The only one which you may need to change or delete is "Readme TPCB105.txt". If you don't delete it, you would fill in the blank or placeholder items near the top and bottom.
However, I recommend that you visit PCBShopper.com first to get instant quotes. Their "Get Prices" form let's you see the results from multiple vendors and it may provide convenient links to them. The info you need to fill in the form is all in the small table in our "Getting...PCB" section. At the time of writing, all items in the table were defaults except the ones in bold.
I will be happy to help if you have further questions. Please let us know how it goes. Best wishes, Steve
January 29, 2023 - 06:01 am|
I'm from Italy. I want to build your heather tracer; I downloaded all the files for commercial production of the PC board from your site.
Can you name any online producer of PC boards that can accept my order and the exact files I should attach to the order? Sorry for the trivial questions, but I'm an old-style hobbyist, not familiar with the PC board world...
February 28, 2022 - 08:08 am|
|Hi Soon-Mo, Thank you for your kind message. Yes, the heater regulator board (HRB) is applicable for use with the uTracer6. The HRB operates independently, controlled by the main power and HtrV switches. Dr. Dekker improved the built-in heater voltage resolution of the uTracer6 by lowering the pulse width modulation (PWM) frequency relative to the uTracer3. However, I consider remote sensing to be the most important feature of the HRB. Without that, it's not difficult to find significant errors in the heater voltage delivered to the tube under test, even if the regulator is accurate. Appendix B of the article discusses remote sensing. You can scan for "highly" to find the introductory info. Below that, the "Remote Sensing Arrangements" section covers it in more detail. |
February 28, 2022 - 12:25 am|
First of all, thanks for sharing a nice circuit. I gave the uTracer3 to a friend and recently bought the uTracer6 kit. I would like to know if HR can be used in uTracer6 as well.
February 21, 2022 - 06:32 pm|
|Happy to help, Rico. Feel free if there is anything else. Cheers |
February 21, 2022 - 06:24 pm|
|Thank you.. Yes I will definitely let you know how this project turns out. I am very curious. Anyway, thanks so far. |
February 21, 2022 - 06:54 am|
|Hi Rico, The 1N4148 will be fine for this. By the way, please let us know how all this works out for you. |
February 21, 2022 - 05:47 am|
|Good to hear. I am going to try this in combination with the XL4015.|
Concerns the flyback diode. What would you suggest? Is a 1N4148 sufficient, or is it better to use a 1N4007? The Coil Resistance of the relay used is 810ohm.
February 20, 2022 - 07:28 pm|
|Hi Rico, Very good. You probably know but the cathode of the diode (marked with a bar) should be on the positive terminal. At the terminals labeled Heater on the main board, the left one (closest to large cap, C6) is positive. You are correct that it is "nothing more than the relay coil with a diode in parallel, connected to the connector where the filament is normally connected". You would NOT want an electrolytic cap across the relay because the current spike from charging the cap could blow MOSFET driver, T3.|
Yes, the XL4015 switching regulator module can replace the original XL4005. This was kindly reported by Gary K., posted below, May 01, 2021.
February 20, 2022 - 04:12 pm|
|Okay thanks for the detailed explanation.|
I will then test with an 18V relay. And indeed with a diode across the coil of the relay.
Then it is nothing more than the relay coil with a diode in parallel, connected to the connector where the filament is normally connected?
Perhaps an electrolyte is superfluous.
Then there is the XL4005.
Am I understanding correctly that the XL4015 can replace the XL4005 without any problem? Do you have experience with this? Since the type you specified is often no longer available, but the XL4015 is.
Add your comments here...
Please note that “µTracer” is Ronald Dekker’s trademark and there is no commercial use intended in this educational article. We are grateful to him for allowing us to use it here.