Comments for page: uTracer-HR You Can Build

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Posted by Steve L. January 11, 2021 - 02:39 pm
Thanks, Markus. The 2-pole DC-type jacks were needed to support a second channel, required by the remote sensing feature of the heater regulator board. That eliminates the voltage drops in the heater and ground connections. Quoting from Appendix B:

"A highly accurate heater voltage source would be of little value if nothing were done about voltage drops in the heater and ground connections. With the connectors and wires, itís pretty easy to find 0.1Ω; or more. At 3A, that drops 0.3V or about 5% of 6.3V, which is way too much error from that one contributor.

Hence, I decided that remote sensing for the heater supply is necessary. With remote sensing, the regulator sets the voltage at the socket, rather than at the regulatorís own output. It does that by using separate wires to bring back the voltages at the two heater pins. Very little current flows in these sense lines, insuring that the regulator is seeing what is actually at the socket pins. Notice, however, that this implies that two separate lines must be connected to each pin: drive and sense. That requires two-pole connectors."

By the way, for convenience in connecting lab equipment to the tube, the red banana jacks are also provided for each pin. Those also use the sense lines, making it easy to measure the tube's pin voltages without error from voltage drops.

Posted by Markus January 11, 2021 - 02:01 pm
Cool report!
Why did you use DC-type jacks for the tube pins and the voltage sources and not banana jacks?
Regards,
Markus

Posted by Steve L. September 16, 2020 - 06:57 am
Hi Hoby, Thank you for your kind message. I'm not sure what you mean by, "I need bin hex files" but I will be happy to help if you have further questions. If you are looking for files such as schematics and panel art for this project, the download links are at the end of Part II of this article. Best wishes for your project.

Posted by hoby September 16, 2020 - 06:17 am
Congratulations on successful article work. I will do it as a hobby. I need bin hex files. thank you. elektordu@gmail.com

Posted by Steve L. March 24, 2020 - 11:05 am
jgxbos (Juan) had also posted his question below in the Google groups uTracer forum and I followed up by posting my 3/22/20 reply from below, there as well. In his kind thank-you note there, he also mentioned that he wanted to modify the heater regulator circuit for other and variable voltages. Here are a few thoughts on that. (I didn't put this there because the thread was about something else.)

Thoughts about modifying the heater regulator board (HRB) for other and variable voltages:
(Please refer to the HRB schematic.)
  • The Loop Integrator compares the feedback from the voltage dividers to a 2.5V reference and steers the output to make the feedback voltage become 2.5V.
  • Adding a 2.5V output setting is easy because no divider at all is needed---we want the output steered directly to the reference voltage; no adjustment needed. However, the Loop Integrator depends on the source resistance of the feedback being about 20Kohms. So instead of directly connecting the Heater Sense (HtrSen) line directly to pin-3 of IC201, we need to insert a 20K 5% resistor. Caveat: Loop stability has not been tested in this condition.
  • Note that the lowest possible HRB output voltage is 2.5V in this circuit.
  • To get variable output, you can add another divider (or replace an existing one) as shown in this schematic: http://www.tronola.com/misc/uT_Heater_Reg_Var_output_example_1800.gif
    Please bear in mind that this is a brief effort and should be considered experimental. In this concept, I added the Fixed/Variable Select switch which lets you choose the (front panel) 2.5-15V Adjust pot, R215. The 15V max is approximate and can be increased modestly by decreasing R217. The max must be well under 19V though. I hope this is what you had in mind but if not, please let me know. Questions and comments are welcome

Posted by Steve L. March 22, 2020 - 07:20 pm
Hi jgxbos, The value of R211 should be 2.7K. Sorry that the Mouser part number in the BOM is incorrect and should be CF1/4C272J. I will correct that shortly. In case you don't have a 2.7K 1/4W resistor, I will be happy to mail one to you, assuming it's in the U.S. You can email me the mailing address. My email is found by clicking the About button on the Tronola Home page. By the way, you are correct that the photo shows the multiplier color differently---there was later work on improving loop stability and the photo was taken before that. Sharp eyes! I will try to correct the photo as well.

C207 is discussed in Part III of the article. (You can search for "C207".) It is important because C207 improves stability. It should be soldered on the bottom of the heater regulator board as shown in Slide-25 of the Gallery linked from that same page.

Thank you for reporting the BOM error! I will be happy to help if you have any other questions or problems.

Posted by jgxbos March 22, 2020 - 05:18 pm
Hi Steve Lafferty

I am mounting the "crazy acccurate" heater supply (Thanks Ihor for the PCB), and I have two doubts:

1) The value of R211 is 2.7K or 270Ohms? In the BOM, the mouser reference (CF1/4C271J) match to a resistence of 270 Ohms, but in the schematics R211 have a value of 2.7K. Moreover, in the photo of the pcb mounted in tronola, I can see the color code of the resistence R211 and seems red, violet brown (270 ohms). What are the correct value for R211?

2) I can't see the position of C207 (0.22uF/50V) in the PCB of heater supply of tronala, is a mistake? I have to solder C207 out of pcb? Is important C207?


Regards

jgxbos

Posted by Steve L. July 08, 2019 - 01:50 pm
Hi Brice, Thank you for posting the kind follow-up. I'm very pleased that we were able to clear up the problem. All the best.

Posted by Brice July 08, 2019 - 01:32 pm
Hi Steve.

Thank you very much for your insight and for taking the time to investigate this matter.

Yes the MaxiPreamp does not give current readings like the Amplitrex so we don't know in what condition the tube is tested at.

I've done your test and indeed, having the Vg around 1V gives you more accurate results.
In this case, there is nothing wrong with the Utracer design, but rather with my interpretation of the results.

I am going to play with the MaxiPreamp and try to find the operating point but I suspect it is adjusted for each measure while ours is static.

I agree with the PS supply upgrade. I just didn't know it would impact small signal tubes too, with such small heater current demand.

Thank you again Steve.
Brice.

Posted by Steve L. July 07, 2019 - 09:26 am
Hi Brice, I have read your thread in the Google Groups uTracer forum, titled "Low GM test results," under the handle, "BHdeC" and can see that this issue has troubled you for some time. Sorry for your frustration but I think we can soon get to the bottom of it. Please refer to the chart seen here: http://www.tronola.com/misc/ECC83_Gm_chart.png (You might want to right-click the link and open it in a new window.) The line labeled "S" is Gm and the scale on the right is in thousands of micromhos. For example, at 1mA, it has about 1550umho. Notice that at 0.3mA, it has only about 800umho. So operating current is a critical parameter in tube testing. By changing the current, you can get vastly different results.

The MaxiPreamp tester manual does not make clear what the conditions of the Gm test are but as mentioned before, they may be much higher than the 0.3mA you mentioned. The other tester mentioned in the thread was Sencore MU-150. I will say that it seems to be a cut above some others from the old days but still the supplies are for the most part, unregulated and control over operating conditions is limited. It attempts to hold operating current to a fixed value of 2, 7 or 25mA. At the lowest of those, the ECC83 chart shows Gm at about 2130umho, so that may explain the lower readings you are seeing with the uTracer.

Right now, I'm plugging-in a JJ ECC83 into the uTracer and running a quick test. Using a chart of plate curves, I see that, at Va=200V, I need to set Vg= -1.1V to get about 2mA to match the Sencore. Running the quick test gives Gm=2480umhos, which exceeds the Sencore reading. On the VTA, with the same voltage settings, I get Gm=2270, so the uTracer reads about 9% higher in this case. Also for those settings, the uTracer gives 2.08mA and the VTA gives 2.09mA. In case you're wondering, the VTA shows that Vg= -1.16 gives Ia=2mA for this tube and Gm=2217 there.

I think you can see that the discrepancies you are seeing can be explained by the different test currents and different tester methodologies. By the way, the heater supply issue in the uTracer is a real and known problem, not just for power tubes. It's essential to power the heater from an external supply, as shown in the article. I will be happy to help if you have further questions. Thank you for diligently pursuing this issue. Doing this kind of crossing-checking and investigation helps everyone understand their instruments and how to use them to best advantage.


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