July 06, 2019 - 10:16 pm|
I've posted my findings to the Utracer Google group forum. Ron is aware and looking into to.
But definitely, the lowest the measured current is the worse the precision is, to not relevant anymore. Cheers.
July 03, 2019 - 08:34 am|
|Hi Brice, Thank you for the kind comments. It's true that the accuracy of the µTracer may be reduced for some low-bias tubes but for the 12AX7 running at the databook condition of 0.5mA, we found that the gm reading was within 2.4% of our own VTA. The Rp reading was 3.6% off in the other direction, so mu was only about 1.2% off relative to the VTA. Regarding the MaxiPreamp tester, I notice that it seems to operate the tube in a peculiar set of conditions and it's unclear whether it is measuring true gm and mu. |
For example, the manual conflates the terms, gain and mu, then shows a chart which appears to list values of external Rp used in the gain test. The problem is, mu is supposed to be the voltage gain for an infinite external plate resistor. For the gm measurement, it's not clear what the conditions are and to be able to compare the reading to the µTracer, one would need to know them. Are they the same as the Gain measurement chart? If so, one can only choose 1, 2 or 4mA, so the gm measured would be very different from that seen by the µTracer, operating at the value you mentioned of 0.3mA.
Too often, we speak of a tube's gm as if it's a constant, when in fact, it's strongly dependent on operating conditions. Whenever I speak of a tube's "ideal gm," it's based on the specific conditions given in standard databooks. I really appreciate your bringing up this interesting topic because it seems we are only starting to emerge from the dark days of tube testers of yore which purported to test gm but didn't operate tubes at standard conditions.
June 30, 2019 - 06:32 pm|
|Very nice article. Excellent source of information.|
I've build one on these UTracer and I am very please with it.
It works great for most tubes with decent emission.
However, for low current tube, typically 12ax7 with a static emission lower than 0.3mA, the gm and mu values are totally off, compared with a MaxiPreamp tester.
This is a problem for me.
June 14, 2019 - 05:37 am|
|Hi Josh, Your implementation of the µTracer looks really nice! Thanks for the link and tips for using the Bluetooth and USB modules. |
June 14, 2019 - 05:27 am|
|Hi Steve, I like your tube tester! One thing I would consider is using USB and Bluetooth instead of the serial port connector. I have my uTracer working with both.|
You can see my tips here: https://www.dos4ever.com/uTracer3/uTracer3_pag9.html
Search for “2018 Josh” and the notes with photos of my build will pop up. You’ll see I also designed a regulated power supply so I can use it with an IEC socket.
June 06, 2019 - 06:01 am|
|Well, I have the M107 too but use it rarely. For tubes there is not enough current capability. As heater supply for the uTracer I use the model PL550-1 Trygon. Max current is 1 A not sufficient for big tubes. There I have other power supplies when necessary. |
June 05, 2019 - 06:47 am|
|Hi Sven, That seems like a great way to power heaters for the µTracer. Doing a quick search for Systron Donner precision lab supplies turned-up the M107 here, so I see that they made some very impressive supplies! Thank you for your kind comments. |
June 05, 2019 - 05:03 am|
|Hi Steve, excellent well done work. As also owner of the uTracer I made it simple and use an vintage external analog linear DC precision lab supply from Systron Donner. This allows setting voltages precisely with digit indicating rotary switches down to mV range. There is also an indication meter for the heater current. Thus I can easily and precisely set the heater voltage to +- deviation in order to measure the tube quality under reduced and over heating. And, there is no problem measuring the voltage with a good DC voltmeter if necessary near the tube under test. |
June 01, 2019 - 05:58 pm|
|Please feel free to add any comments or questions you might have. No sign-up is needed. |