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3D tube curves using an el'Cheapo tube tester
3D tube curves using an el'Cheapo tube tester
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Old 22nd July 2019, 05:52 AM   #1
Ufudu is offline Ufudu  South Africa
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Default 3D tube curves using an el'Cheapo tube tester

This may belong in the 'Test Equipment' section, but I thought I might share a 3D graph of a 6CA7 transfer curves I measured. There is beauty in it's simplicity!

I put together a concept tube tester based on a RPi and Microchip PIC to make these measurements. (Total cost without the RPi screen and anode transformer is about USD 80). Total measurement time (for 6500 data points) was 40 seconds, excluding tube heating time.

The original purpose of the project was to build a self contained tester that was not tied to a laptop/computer, that was fast, let the tube reach normal operating temperatures (ie not pulsed), but still gave full access to the raw measured data so it can be manipulated with Python script.
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Old 22nd July 2019, 06:46 AM   #2
rankot is offline rankot  Serbia
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Very nice! Now you just need something to make Spice models using your data.
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Old 22nd July 2019, 07:27 AM   #3
petertub is offline petertub  Sweden
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3D tube curves using an el'Cheapo tube tester
Very interesting.

Will you share the design with us ?
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Old 22nd July 2019, 07:54 AM   #4
Ufudu is offline Ufudu  South Africa
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And here is the 6CA7 in ultralinear mode...
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Old 22nd July 2019, 08:17 AM   #5
Ufudu is offline Ufudu  South Africa
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Quote:
Originally Posted by petertub View Post
Very interesting.

Will you share the design with us ?
Sure, no problem, however this is not something new or original. It's just that it has been cobbled together using commonly available parts.

I use a classic tube measurement method of using a rectified voltage for Va, (so Va is being swept twice every mains cycle) then measure the corresponding Ia/Is over the first half of the mains cycle (5ms in this country).

The PIC can take 65 measurements (Va and I) in 5ms and that gives you a single trace at a Vg determined by the RPi.

Vg is generated by a simple negative voltage amplifier that takes the reference from a DAC controlled by the RPi. This gives a range of 0 to -100V.

Ia and Is are measured with a series resistor and opamp current amplifiers. The amplifier gain has 3 settings that gives an effective range of 1-300mA.

A sync pulse is generated close to Va=0 to start the measurements. These are performed as fast as the PIC can and stored in an array.

The RPi is coded in Python. I use a 8 GPIO pins from the RPi to communicate with the PIC. The PIC does the Va,Vs, Ia,Is ADC conversion and transfers the array to the RPi.

After that the post processing of the data is done using standard Python library routines (e.g. matplotlib and numpy), or use a variety of programs to extract Spice parameters (eg extractmodel (windows based)) or the curve matching application by Dmitry Nizhegorodov.

The PIC code is in C (C-- in my case...) haha

If anyone wants any further detail let me know.
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