uTracer (Micro Curve Tracer)

[Guy77]

Thanks for the update Martin.
Regarding the 3D image you posted of your uTracer in your post from Jul 06 2022, did you use the EasyEDA pcb design software here?

Thanks, Happy holidays.

Guy

[martin manning]

Hi Guy, KiCad, v. 6. Same for the Low/+ Vg board a few pages back in this thread.

[martin manning]

Most data sheets have a point at Va=250V/Vg2=250V at some Vg1, with the nominal Ia, Ig2, gm, ra, and mu listed. Testing at this point shows how close a tube is to the nominal spec, and in comparing results, how closely matched tubes are to each other. This test point usually gives good results for matching purposes, even though 250V is relatively low compared to the actual operating conditions in most amplifiers. For matching pentodes, plate current (Ia) and transconductance (gm) are usually the most important parameters to look at, with anode resistance (ra) and amplification factor (mu) being the least important.

In this example a (supposedly) matched set of four 6L6GC are tested at the conditions shown on the data sheet:

Datasheet.png

Voltages and nominal Ia, Is, gm, and ra readings from this point are loaded into the QuickTest form, and the the tubes are tested to get Ia and gm readings for each tube. I am using Ihor Smal’s uTracer JS interface (described above in this thread), which uses a "double acquisition" mode to bring the actual test voltages into agreement with the specified Va and Vg2, eliminating any hand iteration. Collecting the data takes time, since the uTracer initially needs 30-40 minutes to stabilize, and each tube must be allowed to warm up for 5 minutes or so. All tubes must be treated similarly for the most accurate results.

QuickTests.png

A pair of tubes can be selected for a close match in both Ia and gm, or if four tubes are required, pairs can be arranged to produce the best overall balance, typically this means placing the tubes with the lowest and the highest Ia in the set on the same side of the output stage. Averaging and comparing the Ia and the gm for the chosen pairs shows the overall match is quite good. Note that if a "half-power" mode (two of four tubes switched out) is used, it is best to have four closely matched tubes installed.

ResultsPairs.png

Where exactly is this test point with respect to the actual operating area on the characteristic curves? A full trace at 250 Vg2 shows that the test point is more or less in the middle of the curves, but if the actual operating conditions are 450 Va and 450 Vg2, a scaled idle point will be located approximately at A, assuming a typical bias condition, and a well-placed load line identifies points B and C, of which B might be more centrally located. Testing at the original point or at B probably won’t make much difference in the end, and the data sheet point has the advantage of the nominal readings being readily available. Point A might be a better indicator of how well the tubes will match at idle.

LoadLine.png

Rather than doing a QuickTest at multiple points, a further step might be to plot a full set of curves and overlay them. The screen voltage can be set to the actual operating voltage, or as high as possible, and the anode voltage can be swept up to the maximum possible (400V). The curves validate the QuickTest results above, and reveal that Article 2 has an odd kink on the left side. Overlaying the curves using the uTJS clipboard, the pairings selected from the QuickTest results above (shown at upper and lower right, below) are confirmed.

FullCurves.png

[martin manning]

Just for fun I threw a PSVane 12AX7 into a recent Amplified Parts order to see what they are like. The box is marked "Classic Series Electron Tube," no special selection criteria are offered.

Of course a single tube is not a statistically significant sample, but here are the results:

- Reasonably good as far as its emission and gm at the 250Va/-2Vg data sheet point, but the current is a bit low.
- The sections are well matched.
- Heater current is 27% high, at 380mA. (Interestingly the last two EH 12AX7's I bought both ran at 400 mA, 33% high)
- In the V1 position in my ODS, in OD mode, all knobs at noon, it has about the same hiss, but significantly more heater hum than the JJ 12AX7S I have been using.
- Sound wise, It's comparable.

[M Fowler]

I still have a kit I never got around to building if anyone is interested in it.

Mark

[tekno808]

Shorts and leakage test installed and working. Running out of room in the box ;^)

In the bottom photo, the 6L6GC has its heater and cathode connected. The leak test is switched in and showing green on the lower LED, indicating no H-K leakage (more than 3Meg resistance H to K). Touching H with one hand and K with the other will switch the LED to red due to the ~1Meg parallel resistance. Even though there is almost 60V potential I can't feel a thing with only 20-30 uA flowing. This circuit can be made to work with uT6, but note uT6 negative voltage supply is -85V, which would result in over 100V from H to K. The -15V supply could be used instead by changing two resistor values to adjust the LED current and the sensitivity. I have extra boards available in case anyone is interested.

Nice addition the Shorts and leakage test
where can i get the pcb or gerber files ?
Thx , Danny

[martin manning]

where can i get the pcb or gerber files ?

PM sent.

[didit]

I still have a kit I never got around to building if anyone is interested in it.

PM sent ..

[mhuss]

I also have a kit I never built (somehow I ended up ordering a second one by accident)

[rogb]

I'd just like to add that I've had my Utracer since around 2015. It was a tough build for me, and I had a few problems that were sorted out by Ronald and of course, Martin Manning.

I have tested many tubes with it up until just before last Christmas, it started playing up.
Long story short, the PCB went back and forth to the Netherlands a couple of times, Ronald tweaked it back into life, and with the help of his wife, Marie Jose, I now have a fully working Utracer!

Bear in mind, I bought it 8 years ago, and those folks are still supporting me and standing by their product. I really wanted to show them my gratitude, after all, who gives free support for 8 years? I sent them some money as a goodwill thank you, they promptly sent it back, minus the postal charges I owed.

If you don't have a Utracer and have a bunch of tubes and the ability to do some delicate soldering, then get one, the Utracer team won't let you down.

[martin manning]

If you don't have a Utracer and have a bunch of tubes and the ability to do some delicate soldering, then get one, the Utracer team won't let you down.

Good to hear Ronald and Marie Jose are taking care of you, Rog. I'll add that some fabrication skills are needed if you want an enclosure with a socket array for the tracer.

Note to anyone picking up an older uT3 kit, depending upon the version there may be some updates required to bring it up to the current configuration. If the construction manual identifies it as uT3+, all good. If it's a uT3, I'd recommend upgrading to the 400V 3+. I've kept a complete log of the changes, and can advise on that. Details are here: https://www.dos4ever.com/uTracerlog/conversion.html

[rogb]

If you don't have a Utracer and have a bunch of tubes and the ability to do some delicate soldering, then get one, the Utracer team won't let you down.

Good to hear Ronald and Marie Jose are taking care of you, Rog. I'll add that some fabrication skills are needed if you want an enclosure with a socket array for the tracer.

Note to anyone picking up an older uT3 kit, depending upon the version there may be some updates required to bring it up to the current configuration. If the construction manual identifies it as uT3+, all good. If it's a uT3, I'd recommend upgrading to the 400V 3+. I've kept a complete log of the changes, and can advise on that.

Good to hear from you, Martin. Your input into the whole Utracer project has been invaluable, I would say.
Yes, the 400v UT3+ is the one to go for, or at least update to.
Hopefully, most folks at TAG will have the ability to build a simple enclosure. It could even be run bare if needed, as long as due care is taken with regard to voltages present, and avoiding costly grounding mistakes ie running the PCB on a metal table

[audiowizard]

Is there someone who might be able to help with my new utracer 6? It passed the tests during construction and I completed the cal section fine but it appears not to function for me testing tubes. I tried 6BQ5 and 12AX7. Also there is no bar graph under quick test and it shows run time test error 13 sometimes.

IMG_2648.jpeg

IMG_2647.jpeg

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IMG_2645.jpeg

[martin manning]

Maybe try the google group? https://groups.google.com/g/utracer

[rogb]

If you contact Ronald and send him the screenshots, I'm sure he will give you some test checks to run.

Is there someone who might be able to help with my new utracer 6? It passed the tests during construction and I completed the cal section fine but it appears not to function for me testing tubes. I tried 6BQ5 and 12AX7. Also there is no bar graph under quick test and it shows run time test error 13 sometimes.
IMG_2648.jpegIMG_2647.jpegIMG_2646.jpegIMG_2645.jpeg