Hello to all members of this forum, my name is Fernando and I'm from Portugal. This is my first post on this forum. I wanted to ask for help in building a simple tube tester based on Arduino like this one: *** A simple Arduino Based Gm,Rp,Mu Tube Tester by A.Galavotti *** I did a test assembly and encountered the following problems. I only get the correct readings for Vg, Va, and Vf, but they are much below the input value. As for Ia, I wanted to ask for help on where to connect the A2 port of the ADS1115 to the analog part of the circuit. My goal is to test normal tubes ecc81,82,85,88, El84, el34, etc. I appreciate anyone who can help. Thank you, Fernando
Tubes vary a lot and different readings is normal. You need to start with a known values to check your design. Normal tubes, I recognize most of those numbers but it has been over 40 years ago. You might want to do some research before going to far. Also there were many designs for tube testers published in the 60's - 80's time frame.
So you mean vacuum (radio...) tubes? Triodes only, or also pentodes?
What's your electronic background? I started around 1963 by decomposing radios and building tube amplifiers.
As a radical simplification I'd use a fixed HV supply and an (exchangeable) anode resistor, and a cathode resistor to produce the negative grid voltage. Both resistors can be selected according to the tube data sheet.
Then the grid voltage can be modified easily by adding another (parallel) cathode resistor. Or a programmable constant current source can be used to adjust Ia and determine the resulting Vg and Va. This will restrict active parts to usual (MOSFET) voltages. The Ia can be measured as Vk/Rk. Effective Va has to be calculated as Va-Vk.
Please give a link to the data sheet containing A2.
Why don't you start with the 10 bit Arduino ADC and MUX?
Connect the ADC inputs to anode, cathode and grid, differential reference voltage to GND (HV-).
Then you'll have to wire the pins according to the tube under test. How do you want to accomplish that?
1 Like
Some people call them valves! But for emission tests, all grids can be tied together for testing.
I know how to test valves, I have several old tube testers, but I want to build a digital one that shows me the GM, RP, MU. My main question is where to get the Ia reading, because the diagram in the link I followed refers to a direct heating valve.
Hello fapam
A tip from me:
Visit your local radio amateur club.
There you are most likely to find someone who still works with tubes and can perhaps also give you tips on measuring.
Read my #3 for details. If you don't understand something then ask.
The diagram you gave seems to assume that Ig is negligible and Ik is a satisfactory approximation of Ia, and they only measure Ia through a pair of 1R shunts to GND.
You could follow a similar logic and connect a shunt between the cathode and GND, and measure the voltage across the shunt.
I do think the whole setup as shown as a high potential of quite spectacularly blowing up an Arduino and/or ADC board. It should work in principle, but there's virtually zero protection against mishaps.
Such a "shunt" is typically used to (also) generate the negative grid voltage - a win-win construct.
Yes, good point; the same Rk can be used for current measurements and to lift the cathode above G1 potential. It's usually decoupled with something like 470uF in a typical audio application. But you've known this for decades, already...!
Vg (input -8.3v, reading -8.3v), Va (input +200v, reading +9.34v), Vf (input +12v, reading +0.53v)
I think the ADS1115 module is having issues, so I'll order another one to redo the test. It's the first time I'm working with Arduino, so everything is new to me, sorry for my ignorance and thanks for the responses. If any expert can make the necessary changes or suggestions directly in the schema, that would be ideal.
Yeah, that's not right. The measurement setup looks like simple enough resistor dividers that are fed to non-differential inputs on the ADS1115. Ensure that you've got the ADS1115 in the correct mode; if you've got it inadvertently setup to measure differential pairs, you're in for some weird outcomes. There may be other issues, but in all honesty, I'd prefer to see a single schematic with regular symbols (and square boxes for IC's/modules) that's a little easier to read in one go instead of the multiple screen captures with module pictures.
To come back to the Rk thing discussed above with @DrDiettrich - for a tube tester, a proper independently variable -Vg supply is preferable to make Rk and -Vg independent from each other. It's still possible to infer something from a grounded-grid setup, but it's not ideal. Anyway, OP's setup already uses this approach, so it's kind of a moot point.
Get a scope (or earphone) and check that you don't have AC modulation (hum) on your DC signals.
1 Like
This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.