OT^4: an Over The Top Tube Tester

Robert L

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Apr 7, 2019, 7:01:15 PM4/7/19

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OT^4

an "Over The Top Tube Tester"

A tiny bit of history...

I’d previously modified Marcin Saj’s Pro Tube Tester and Healer adding user adjustable current limiting, cathode current metering, and switch selected metering of the high voltage supply output and operating anode voltage. Here's a photo of my modified Marcin Saj Pro Tester showing 3.2 mA cathode current with 139 V across the tube under test:

IMG_20170519_183548194 (Large).jpg

The OT^4 was hatched in casual conversation with Nick S. about tube conditioning near the end of 2018… Why not build an Arduino shield with similar capabilities to the tester above, but under processor control? Why not use tube carriers from Marcin’s nixie shield for Arduino and his more recent tester for increased flexibility and ease changing the tube type? It would be an interesting device and a good portion of the circuits were already designed, LTSpice modeled, built and tested for the modified tester above.

The new tester would be an Arduino Mega 2560 shield where the Mega 2560 was a bit of overkill, but large enough to provide a stable base.

Fast forward a few months…

This is the new tester – the OT^4. The IN-12 shown below is running cathode 2 at 75% of max data sheet cathode current with 2.3 mA measured cathode current at 138 V across the tube. The test is 95 seconds into a 600 second conditioning cycle.

OT^4 in operation:

Two Marcin Saj tube carriers along side the OT^4:

(yes... 2 blue wires... sigh...)

The OT^4 hardware is capable of sourcing from 165 V to 240 V using a Taylor 1364 HV supply; programmable cathode current limiting ranges from 200 uA to 40 mA. Read back of actual cathode current, output of the HV supply and anode voltage are all at 1% accuracy. Settings for voltage and current limit are a tad granular due to the 256 step digital pots used in the control circuits… but more than sufficient for a tube conditioner!

Firmware uses a data structure to capture parameters for each tube type… data book operating current for each cathode, limits on tester applied voltage and current for that tube type, defaults for conditioning cycle time and such. Conditioning current is run-time adjustable and is specified as a factor of the data book max current using the rotary encoder. The user can also run time select which cathodes to condition. The Arduino serial port is used to output a log of tube operating parameters collected at regular intervals during conditioning.

Here’s a short movie of the tester in operation:

https://photos.google.com/share/AF1QipOTrt8kheRuywKuUz5s0G4NB3qovVrsgUnlL5ea-g7X7xDBfQSQwDAPg0yOtvRAKw/photo/AF1QipNmtFnD6uHM0XpM9R_AJn59wAmpjKXkbTypL9H_?key=SkluWDh0ay14ZUVFamtXT3JhemlNSnFzd1dBZ0xn

And below, a section of a conditioning log… Note the programmed HV supply starts out at the maximum available 240 V to help “reluctant” tubes ionize, then almost immediately drops to a lower voltage once the tube conducts – either the low limit at 165 V, or just a few volts above the required anode voltage when the anode voltage rises above 160 or so volts for larger tubes operating at higher currents. Adjusting the HV supply for minimum required voltage during operation prevents needless heating and stress on the HV supply and OT^4 pass element.

OT^4 Rev 1.00: f/w testing
Cathode, runTime (sec), Factor (%), Iset (mA), Vset, Vhv, Vanode, Ianode (mA)
Log, tube type, IN12A
Log, Enter Select Cathodes
Log, Enter Condition Cathodes
Log, Conditioning Cathode: 2
2, 0, 0.75, 2.25, 240.00, 239.75, 138.98, 2.30
2, 26, 0.75, 2.25, 165.00, 164.18, 138.13, 2.30
Note, try a few different conditioning factors
2, 56, 2.00, 6.00, 165.00, 163.61, 157.38, 6.01
2, 86, 1.00, 3.00, 165.00, 164.18, 141.53, 2.97
2, 116, 0.50, 1.50, 165.00, 164.18, 134.17, 1.50
Note, and back to 0.75 of rated cathode current
2, 146, 0.75, 2.25, 165.00, 164.18, 138.13, 2.32
2, 176, 0.75, 2.25, 165.00, 164.18, 138.13, 2.30
2, 206, 0.75, 2.25, 165.00, 164.18, 138.13, 2.30
2, 236, 0.75, 2.25, 165.00, 164.18, 138.13, 2.30

If it’s worth doing, it’s worth doing well… and maybe taking it just a bit over the top! It’s a hobby… Right?!

Best regards,

Bob

martin martin

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Apr 7, 2019, 7:04:55 PM4/7/19

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Excellent!

I think I want one (need?)

where can it be had?

-martin

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