Finding NIMO: Chapter 1 - The Arrival

[gregebert]

Well, it's been a long haul since this project started in 2017, and today was a major milestone with the arrival of 3 more tubes. They are re-joining their sibling after 5 years.

Filament data was gathered and verified to be consistent with the first tube. so tomorrow I can begin powering-up each one individually and gather more data before I dare take a chance and plug all of them into the clock. Besides, 3 more just landed at the airport and are waiting to be delivered.

More details to follow in the coming week, or weeks (plural) if there are problems.

[Audrey]

How much did all this cost you? O.o

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[gregebert]

It's not a dollars-and-cents answer. I've spent hundreds of hours, and a fair amount of money on the research, design, debug and development to get this very complex design working. At a future date, there will be a limited number of tubes made available for sale by the co-sponsor of this project and I will be making the design available for others to download (free of course) once it's been proven to be stable and reliable with 6 tubes. I will forewarn that the board has some challenging SMT parts, and requires the FPGA to be programmed. It's just unfortunate that NIMO tubes are so rare.

[Dekatron42]

Does your design run them multiplexed or direct drive?

/Martin

[gregebert]

Direct-drive. I never considered multiplexing them, though it might be possible due to phosphor persistence.

Now that I've powered-up 4 tubes and gathered more data, I'm seeing tube-to-tube differences in brightness which is not only visible, but also measurable via anode current. That could complicate multiplexing.

[Kevin A.]

Excited to see these light up!! 

On Sat, Sep 24, 2022, 2:05 AM gregebert <greg...@hotmail.com> wrote:

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[Dekatron42]

The datasheet IEE_Nimo_808.PDF that can be found on the internet mentions "Time sharing" (multiplexing) and I have just glanced at it to see if it would make the circuit simpler.

Is the brightness difference a problem?

Is it possible to compensate for it by using a filter or by using a series resistor with the anode or by any other means?

Thanks for sharing the data on the heater/anode measurements and the project as a whole!

/Martin

[gregebert]

When driver ICs were relatively expensive, it may have made sense to share across tubes by pulsing the filament (cathode) bias as shown in the datasheet. Theoretically, you can compensate for differences in tube brightness by adjusting the bias uniquely for each tube. I dont know if you need to increase the anode current (usually by running at a higher voltage) to compensate for the multiplexing's impact on brightness.

I'm going to wait-and-see till after I get the next 3 tubes to see if there's a real problem that needs to be fixed.