Modifying a tube tester into a curve tracer (oscilloscope X/Y outputs) ? - diyAudio
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Old 19th October 2006, 06:31 PM   #1
bembel is offline bembel  Europe
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Default Modifying a tube tester into a curve tracer (oscilloscope X/Y outputs) ?

Hi all,

I'm looking for a simple/cheap way to plot tube curves, and have a spare hickock I177b transconductance meter sleeping there.
(here a link to the schematic ----> http://www.suertenich.com/html/tester/i177.gif )
I also own the expansion box MX949 that allows any pins connection

My basic idea is to apply a stairsteep signal to the grid, while a rectified AC is sent to the plate ( also used as X deviation )
Y would come from a sense resistor.

It would also be nice for a future project to reuse those outputs to feed a computer interface.

Any Suggestions ?
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Old 20th October 2006, 09:02 AM   #2
coresta is offline coresta  France
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Bonjour, le principal pb est de trouver un générateur de tension en escalier . Le schéma du tektro 570 se trouve et donne une idée de la complexité du système . J'y réfléchis également !
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Old 20th October 2006, 09:16 AM   #3
EC8010 is offline EC8010  United Kingdom
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coresta, pertinent though your comment is, please remember that this forum is designated as English-speaking.
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Old 20th October 2006, 09:21 AM   #4
coresta is offline coresta  France
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Ok Mr EC8010 Just told i was working on the same idea as i discovered that the main difficult of the Tek 570 was the tubed steps voltage generator
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Old 20th October 2006, 09:33 AM   #5
EC8010 is offline EC8010  United Kingdom
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Sadly, curve tracers are difficult. Often, you see equipment that is overly complex, but getting the complexity of a curve tracer down to manageable is difficult. Every time I look at the idea, I turn away...
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Old 20th October 2006, 12:27 PM   #6
bembel is offline bembel  Europe
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Hi, my first attemps are very impressive !
Topology allready works (tested with low voltage ECC86), not used yet the tester but two DC supplies (logic for stairstep + filaments) and an AF generator (AC sweep).
The curve is clear and very instructive (you can see them moving as filament voltage change for example).

Here's my test sample schematic:
(thanks to Kazuhiro Sunamura (---> http://www.intio.or.jp/jf10zl/) for the idea !)

PS: the spice schematic shows a transistor being tested at the right, I've breadboard it and tested it with both transistors and tubes (ECC86 & ECC83) AF gen feeding a step up transformer for VA >50V, stairstep fed to the grid via a tel line transformer (can provide different bias offsets for the stairs.)
Attached Images
File Type: jpg curve tracer test.jpg (32.5 KB, 805 views)
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Old 20th October 2006, 12:40 PM   #7
Yvesm is offline Yvesm  France
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Hi French and British guys

Same work pending !
For now a very dumb plate voltage and stairstep generator:

Click the image to open in full size.

To be improved a lot, indeed
As is, it allowed me to write an application in VB which can do that:

Click the image to open in full size.

Just using a sound board as an A to D

Yves.
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Old 20th October 2006, 12:45 PM   #8
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Quote:
Originally posted by EC8010
Sadly, curve tracers are difficult. Often, you see equipment that is overly complex, but getting the complexity of a curve tracer down to manageable is difficult. Every time I look at the idea, I turn away...
Actually, once you figure out the retrace blanking it isn't that difficult to implement with a scope -- the second issue of Glass Audio had such a circuit in which grid voltage was stepped with a CMOS clock.

There are, however, any number of data acquisition cards which allow you to both control voltages and measure VI -- and dump the data into excel for analysis -- the beauty of employing one of these, even with an inexpensive tube-tester (like an Eico or Heathkit) is that all the nasty wiring is already done. once the data is in excel all the useful information can be calculated -- and you can compare multiple devices for matching.

with regard to a "Sound Card" -- better make sure you put some protection on the output or you will quickly fry the caps -- and sound cards don't take well to DC inputs anyway -- if you can get a copy of the July 2006 Dr. George Steber had an article in the ham radio magazine QST describing a sound-card based IV tracer. A local ham radio club in France should have a copy.

jack
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Old 20th October 2006, 12:49 PM   #9
Yvesm is offline Yvesm  France
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Quote:
Originally posted by bembel
. . .
Here's my test sample schematic:
(thanks to Kazuhiro Sunamura (---> http://www.intio.or.jp/jf10zl/) for the idea !)
. . .
And thanks to you for pointing at (Great link !)

Yves.
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Old 20th October 2006, 01:04 PM   #10
SY is offline SY  United States
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Jack, the soundcard doesn't need to see DC. A good workaround is to apply an offset sinewave with a moderate frequency (say, 500 Hz) to the plate, with the troughs reaching some low voltage or zero and the peaks reaching the max voltage of interest. This can be done with a simple HV amplifier driven by the soundcard.

At the same time, apply a 510 Hz offset sinewave (running from zero volts to cutoff) to the grids.

Using the soundcard input, measure the AC current through the plate/cathode via a small sensing resistor.

If you are generating these waveforms via software, then all the timings are known and a complete set of tube curves can be constructed in about 0.1 second. Running a full second allows averaging for lower noise.

I'm rather software stupid, so this has sat in a drawer for about forever without getting done.
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