uTracer (Micro Curve Tracer)

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JazzGuitarGimp
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Re: uTracer (Micro Curve Tracer)

Post by JazzGuitarGimp »

Hi Martin,

I was not aware that g1 current was specified on published graphs.... Is it on the plate curves graph, or some other graph?

I don't see how a version of the u-Tracer that can go out to 600 or so volts needs to be along the lines of industrial test gear of yesteryear. It shouldn't be that difficult to design a microprocessor-controlled high voltage power supply....


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Lou
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martin manning
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Re: uTracer (Micro Curve Tracer)

Post by martin manning »

JazzGuitarGimp wrote:I was not aware that g1 current was specified on published graphs.... Is it on the plate curves graph, or some other graph?
See on pages 4 and 7 here for g1 current: http://tubedata.milbert.com/sheets/093/6/6L6GC.pdf
JazzGuitarGimp wrote:I don't see how a version of the u-Tracer that can go out to 600 or so volts needs to be along the lines of industrial test gear of yesteryear. It shouldn't be that difficult to design a microprocessor-controlled high voltage power supply...
Take a look at the web log of the uTracer 4, on which work has been suspended. I'm sure there are other approaches that could be used, but I think any such device would be difficult and expensive to produce.
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JazzGuitarGimp
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Re: uTracer (Micro Curve Tracer)

Post by JazzGuitarGimp »

Hi Martin,

Wow, not only have I never seen a datasheet that specifies g1 current, but I've also never seen a datasheet that gives g1 values above 0V.

After skimming the v4 blog, I sense that the biggest roadblock to a higher-voltage model is the creator's (I don't know his name) reluctance to add a transformer-based power supply as well as increasing the size of the product to accommodate said supply. These two tenants (small size and the ability to power the unit from a notebook computer power brick) appear to be very important to him. It's too bad, because even if it took a power supply the size of a small microwave oven to facilitate a version that can test KT150's and 803's up to their design limits, i would be very interested in building one.

Cheers,
Lou
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Merlinb
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Re: uTracer (Micro Curve Tracer)

Post by Merlinb »

JazzGuitarGimp wrote:Hi Martin,
the u-Tracer still isn't quite up to the task of doing R&D in the real world - although bumping the HT supply up from 300V to 400V is a good start. It would be nice to see it be able to go up to 600V plate and screen, or, better yet, 800V plate and 600V screen,
What on earth do you think you would you learn from plate curves that go up to 600V that you can't get from curves up to 400V :?:
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martin manning
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Re: uTracer (Micro Curve Tracer)

Post by martin manning »

What difference does +125V make? Here is some matching data for a set of four Svetlana Winged C 6L6GC, purchased as a matched quad a couple of years ago. A "Quick Test" was run on each tube at the nominal 250V (Va=Vs), Vg1=-14 reference point, and also at 375V and Vg1=-32V. This higher voltage point was chosen to reach approximately 70% of maximum anode dissipation at the highest voltage possible, leaving some margin for the anode and screen voltages to be varied (+/-5%) to get derivatives for Ra and mu. In fact the Ia values are measured at ~394V as part of the test, I just didn't go and read them from the test data matrix.

The four measured values for Ia and gm were averaged for each test condition, and the percentage from the average was calculated for each tube and plotted. The intent is to show the difference between a matching test run at the nominal spec point and at a point closer to actual operating conditions.

The results show gm to be extremely consistent between the two test points (a spread of +/-2%), while the spread of anode current has increased somewhat at the higher test voltage (to ~+6/-4% from ~+3/-3%).

All four tubes have identical "21/2.2" markings on their boxes, presumably from the matching test done by the seller. The average Ia and gm values as measured by the uTracer are 68.3mA, 5.66mS at 250V (95% and 94% of the data sheet values), and 58.0mA, 4.23mS at 375V.
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martin manning
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Re: uTracer (Micro Curve Tracer)

Post by martin manning »

How long do you have to wait to get a stable reading? I ran 40 trials, one per minute, on a 6L6GC and found that its quite a while- at least 40 minutes!

I suspect that the transient behavior is related to temperature stabilization of electrodes other than the cathode, and perhaps in this case the large mass of the beam forming plates and their distance from the heater are responsible for the large swing in the screen current. There is rounding going on, but it's pretty clear that there is an overshoot in current and gm at ~5-10 minutes.

In the end the data shows less than 2% swing in Ia and gm after a 5-minute warm-up but, well, now you know.
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martin manning
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Re: uTracer (Micro Curve Tracer)

Post by martin manning »

Of course some of this drift in measured current could be due to the uTracer itself. How much? Measuring current through a couple of resistors over time reveals that about 1% of the downward drift is due to the uTracer. Not enough to account for all of the overshoot seen in the 6L6GC trace above, but a substantial portion of it. Also, the screen measurement drifts a bit more than the anode, the same as in the tube trace.
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roberto
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Re: uTracer (Micro Curve Tracer)

Post by roberto »

Thank you for your infos, martin!
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martin manning
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Re: uTracer (Micro Curve Tracer)

Post by martin manning »

Posting my panel layout and description here for anyone who wants to duplicate it.
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rogb
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Re: uTracer (Micro Curve Tracer)

Post by rogb »

Martin that is a really superb document. I look forward to building the uTracer and this will be a whole lot easier and better finished thanks to you.
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Jack Hester
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Re: uTracer (Micro Curve Tracer)

Post by Jack Hester »

martin manning wrote:Posting my panel layout and description here for anyone who wants to duplicate it.
That is a really nice document. Well done!

Jack
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rogb
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Re: uTracer (Micro Curve Tracer)

Post by rogb »

I have ordered my uTracer and all (i hope...) the associated parts/case etc. Looking forward to being able to test some tubes in the near future!
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martin manning
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Re: uTracer (Micro Curve Tracer)

Post by martin manning »

Thanks Jack and Rog, I've found this layout to work very well, and it permits plugging up any configuration you might dream up.

Here's an example: I wondered what would happen if I tied the grid to the cathode in a 12AX7 triode (Vg=0 by definition), and specifically if I could see the effect of grid current. The result is below, and in the top-most curve all of the current flowing into the cathode is sensed by the uTracer.
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martin manning
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Re: uTracer (Micro Curve Tracer)

Post by martin manning »

What does grid leak bias look like?

Here's the same set-up as above, with the same grid-tied-to-cathode line above the Vg1=0 line. The grid was then connected to the cathode through 1M, 2M2, 3M3, 4M7, 6M8, and 10M resistors. Adding the 1M shifted the Ia curve down to the Vg1=0 level, but the higher-resistance traces are bunched closely together. This suggests that grid leak bias is pretty hot, and the resistor value isn't too critical for setting the idle point. The typical value seen is around 4M7.
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jazbo8
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Re: uTracer (Micro Curve Tracer)

Post by jazbo8 »

Very cool, and thanks for sharing the 12AX7 grid characteristics. I was wondering if you have a 7199 handy? I am looking for the triode-strapped plate curves for the pentode section, it seems that no one has done one yet...
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