Author Topic: el'Cheapo tube tester  (Read 2017 times)

Offline Ufudu

el'Cheapo tube tester
« on: July 20, 2019, 12:13:29 PM »
After thinking a bit about the comments made here and elsewhere about tube testers I decided to knock together my version of a relatively cheap but versatile tester.
My go-to tester has been the uTracer, mainly because it can provide the LTSpice models, but also it was relatively inexpensive.
However there are a few aspects of the uTracer that could be improved:
1. measurements are pulsed, so the tube never reaches operational temperatures;
2. it is relatively slow, multiple traces can take a long time;
3. it relies on being coupled to a computer running Windows 7 or lower (I am using a iMac in VirtualBox) so this limits flexibility/portability etc.
4. The GUI cannot be customised.

The classic tube tracer uses a rectified 50Hz voltage signal and an oscilloscope set in X-Y mode to provide Va-Ia/Is curves, so I adopted this method since it offers some advantages.

This picture shows Va (blue @ 100V/div) and Ia (yellow) traces



I decided to use a spare RaspberryPi 3B+ as a controller, with a Microchip PIC to do the 'grunt' work.



The RPi instructs the PIC to take a burst of measurements, starting at a sync pulse when Va = 0. The PIC ADC's take about 11us to do a single conversion, so it can take up to 65 measurements (x2 for Va & Ia) in a 1/4 of the mains cycle (5ms).

The RPi code is written in Python and the PIC code in C.

Since the current and voltage measurements are taken sequentially, I interpolate the current data points to get time alignment. Va/Ia curves and Va/Is curves are are taken from different 1/4 cycles.

Each sweep takes 5ms and I take 5 sweeps and average the results. The RPi controls Vg via a DAC and a simple grid voltage amplifier that gives a range of 0 to -100V.



Python has many wonderful data manipulation and graphics functions so you can even plot a 3d version of the tube curves. This is a rather satisfying image!



You can use the data to calculate all the usual small signal parameters at all the operating points, and with some effort (still working on this one) extract the LTSpice parameters.

I plan to build this into a briefcase style tester that should be compact and portable (still needs a mains connection). Things I still need to add is automate the Va & Vs voltage selection, and add variable current amplifier gain control.

Coming back to the original objectives:

1. this construction is cheap on basic hardware. The RPi & PIC and Vg supply is about R800. The Va transformer probably needs to be custom wound with a few taps to give voltage ranges of say 0-100,200,300,400V. Add RPi screen cost & other hardware.

2. it allows the tube to get to a 'normal' operating temperature (depending on your choice of bias)

3. It is fast. 40 traces takes 40s (excluding heating time)

4. The RPi makes it self contained.

5. Linux/Python makes the GUI infinitely customisable. (you actually do not need a GUI. This design does not need a keyboard or mouse, everything can ge set up via the small LCD screen and one encoder wheel. And a BIG RED button to start measurements)










Offline Ufudu

Re: el'Cheapo tube tester
« Reply #1 on: July 20, 2019, 12:27:06 PM »
Sorry, I just realised that the breadboard picture does not show the small screen, encoder wheel and BIG RED button.

I think all tube projects should have a big red button.


Offline Ufudu

Re: el'Cheapo tube tester
« Reply #2 on: July 20, 2019, 02:07:08 PM »
Here is a 6CA7 anode and screen current in 3D...


Offline Tubesocket

Re: el'Cheapo tube tester
« Reply #3 on: July 21, 2019, 08:01:16 PM »
 :popcorn:This is super cool! Well done! Would love to know more!

Online fredeb

Re: el'Cheapo tube tester
« Reply #4 on: July 21, 2019, 09:14:10 PM »
Geez ! You're a genius Ufudu !

Can one operate the device from windows ? I know some other apps that use python are ported for windows .

Would you be prepared to sell as a package ? Perhaps with just 2x sockets , Octal and 9 pin - that would work for most current production tubes .
« Last Edit: July 21, 2019, 09:23:21 PM by fredeb »
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline Ufudu

Re: el'Cheapo tube tester
« Reply #5 on: July 22, 2019, 08:19:48 AM »
Geez ! You're a genius Ufudu !

Can one operate the device from windows ? I know some other apps that use python are ported for windows .

Would you be prepared to sell as a package ? Perhaps with just 2x sockets , Octal and 9 pin - that would work for most current production tubes .

No genius I'm afraid! I remain a hacker when it comes to this type of thing. Other people write programs in C++, whilst I write them in C--  ;D

The tester is self contained, it does not need to be operated from another computer. The RPi runs Raspbian, a version of Linux so all the data files can be accessed via any Mac/Windows/Linux computer for post processing. The Python code is not transportable because it depends on the RPi hardware interface to work.

The idea was not to make a commercial device, there is far too much time/money/NRE involved. As you can see from the pictures, I use matrix board and protoboards, it is not a professional product! (particularly at these voltages!!)

All the components are available in SA off the shelf, and if anyone is interested I can make the code for the PIC and RPi available FOC, but for the rest you will need to get your soldering iron out! The circuit is pretty simple and infinitely variable depending on what voltages/currents you are targeting.

The Python code is not yet complete, and I am still designing a relay switch circuit that will switch in the correct Va/Vs and current amplifier gains automatically.


Offline Ufudu

Re: el'Cheapo tube tester
« Reply #6 on: July 22, 2019, 10:26:07 AM »
And here is the 6CA7 in ultralinear mode  :D


Offline Nchebe

Re: el'Cheapo tube tester
« Reply #7 on: July 22, 2019, 09:26:53 PM »
Really cool!! :clap:

If you really really want to sample V and I at the same time, check out the 16LF1554/9 - dual ADC...

Offline charles

Re: el'Cheapo tube tester
« Reply #8 on: July 23, 2019, 08:47:33 AM »
Congratulations, well done.


Offline Ufudu

Re: el'Cheapo tube tester
« Reply #9 on: December 02, 2019, 05:50:53 PM »
Here is an update on my DIY tube curve tracer.

I have used an Al toolbox as the enclosure. It's bigger than it needs to be but it does the job.

Final voltage and current ranges are as follows:

Anode sweep voltage: 0-150, 0-200, 0-300, 0-400V; Iamax = 300mA
Screen fixed voltage: 100-300V in 10V increments; Ismax = 100mA
Grid fixed voltage: 0 to -100V in 0.1V increments

Measure a batch of Ia and Is for up to 99 grid voltage steps at a fixed screen voltage;
Measure up to 9 batches sequentially (all test parameters can be changed for each individual batch)

Estimated accuracy:

Va 2%
Vs +- 1%;
Ia +- 2% & +- 0.4mA;
Is +- 1% & +- 0.1mA;
Vg +- 0.1V





Offline Ufudu

Re: el'Cheapo tube tester
« Reply #10 on: December 02, 2019, 06:03:52 PM »
I forgot to add:

The tube base is one of Pete Millett's (www.pmillett.com) tube bases. These can be detached & others screwed in.

Alternatively the banana plug connections can be used to connect to a tube socket of your choice.

The on board filament supply provides 2.5, 5, 6.3 and 12.6V dc @ 5A regulated.

Or you can hook up any bench supply directly.

If anyone wants more detail or wants to come have a look-see (Cape Town Area) let me know.

Online fredeb

Re: el'Cheapo tube tester
« Reply #11 on: December 02, 2019, 09:02:28 PM »
I forgot to add:

The tube base is one of Pete Millett's (www.pmillett.com) tube bases. These can be detached & others screwed in.

Alternatively the banana plug connections can be used to connect to a tube socket of your choice.

The on board filament supply provides 2.5, 5, 6.3 and 12.6V dc @ 5A regulated.

Or you can hook up any bench supply directly.

If anyone wants more detail or wants to come have a look-see (Cape Town Area) let me know.

Hey Ufudu - that looks incredible , I'd love to pop round and take a look .

PM sent .
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts