Digit Brightness, Classic Rev6, IN-8 Tubes

6 months 3 weeks ago #12784 by Ty_Eeberfest

Torsten Lang wrote: OK, that confirms that there is nothing unusual going on with the multiplexed tubes.

I agree. Nothing wrong with the drive circuitry or multiplexing code. It's working as designed. The only thing I have left is to question the tubes themselves. Frustrating to not have the thing on my bench so I can see this brightness anomaly first hand!!

Look into it later when the dust is clearing off the crater.

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6 months 3 weeks ago - 6 months 3 weeks ago #12785 by Torsten Lang
Ty, the first reference to the cubic influence of the drive current on the lifetime was in one of the Burroughs documents you linked. Furthermore they write that the tubes the app notes refer to were specially designed for multiplexed operation. I don't know if this is also the case with the IN-8 tubes. If not it would mean that even with the current setup the lifetime could decrease significantly.

BTW, I interpret the datasheet in a way that even if you would go down to 5mA peak it would be too much. The datasheet mentions "10% PWM", in other words these values would be ok for a 1:10 multiplexing but not for 1:6 multiplexing (about 16% PWM).

Jedidiah: If you really want to increase the drive current up to the max. you should also check out if the "software based" HV circuitry is capable of providing the needed current. You could perhaps try this out by removing the tubes and put a load resistor between the 170V and GND and do some measurements.

To be honest I'm not a big fan of these software solutions. I have some trouble with Jürgen Grau's ("Mr. Nixie") ITL 1A clock which after some time of flawless operation now starts to flicker when maximum brightness is requested. Furthermore from the beginning the DC/DC konverter was anything but silent but begins to chirp as soon as a certain output power is needed. To a certain degree I understand why this is done because for the extremely high ratio of output to input voltage you would normally need a transformer based converter like in the Blue Dream which has a customized transformer.

BTW: It's OT but in the old publication I found a nice solution for the series connection of two neons on 170V - a thing that failed multiple times on my Blue Dream clock once the neons start to age.
Last edit: 6 months 3 weeks ago by Torsten Lang.

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6 months 3 weeks ago #12788 by Ian
One other thought came to mind. Have you run this clock for a few days? (I presume so, the conversation we have had has been a long one).

I have seen in the past that tubes reach their operating brightness after a few days of operation. I can't honestly swear that this is true, it's an observation that I haven't really taken much notice of - it was just "one of those things" and might be entirely imagined on my part.

If there is any truth to this, perhaps others can confirm.

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6 months 3 weeks ago #12789 by Jedidiah
Ty, the IN-8s I have are supposed to be NOS. Just looking at them, I have no reason to believe they are not. I do have another IN-8 tube which I purchased from a different source, so I swapped this one into the clock, but the behavior appears the same.

Ian, I will let the clock run continuously for a few days just in case..

I have a bit to digest here with all of these much appreciated comments. So will be back in a day or so.

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6 months 3 weeks ago #12790 by Ty_Eeberfest
Torsten, I agree with your interpretation of the IN-8 data sheet. I wonder how conservative the pulse specs are. I haven't had any problems with Ian's clocks wearing out tubes prematurely, as far as I can tell at least, not that my experience proves anything. I have always preferred direct drive and the stuff that's come up in this thread just validates my preference.

For my own one-off builds I use Supertex HV shift registers for directly driving the neon. Since they are one-offs cost is not a major concern. For HV generation I use a circuit that's been floating around online for ages that is based on MAX-1771. It's probably overly simplistic but it does do a decent job of providing a few 10's of watts of stable +180. I definitely prefer it to a software solution but again I don't need to control costs.

Thanks for linking that Miller publication! Flipping rapidly through it, it looks very interesting, I plan to spend some time with it soon. Those old circuit examples may not be very useful today but still I think there's a lot there to be learned.

Look into it later when the dust is clearing off the crater.

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6 months 2 weeks ago - 6 months 2 weeks ago #12791 by Jedidiah
Well, after all of this, I agree that the clock is running as designed and that there is nothing that can be done in the software to increase the brightness (sorry about my learning curve for multiplexing!). A few more comments and thoughts:

1. I do have an Ian Arduino Shield IN-14 clock that has been running for years at this point. It is also a multiplex design. Putting the IN-8 clock side by side with it in a somewhat darker room, I cannot detect any difference in brightness. So for a clock located away from bright light, it is just fine as is.

2. I've now gone ahead and increased the target voltage of the IN-8 clock from 180 to 195 and that has increased the brightness a noticeable amount. I now show 4.25 volts dropping across the 3k resistor for a current of 1.42 ma. How much higher can the voltage be pushed with this design?

3. I do wonder a bit about the tubehobby datasheet. There are several "recommendations" here. For example, the one about keeping anode current to 1 ma at 180 volts. Is this referring to supply voltage? Also, Nixie tubes seem quite tolerant of higher currents over short periods of time (hence Ian's tube healing mode). Any additional clarification here would be appreciated.

Also, I've been running an IN-18 clock from GRA&AFCH for about 8 months now. It's a direct drive clock with a 180 volt supply. The interesting thing about it is that the anode resistors are 1k. I measure about 5 volts across them which is also 5 ma. The sustain voltage for these tubes is about 130 volts. So GRA is pumping about 50 ma through the tube with a 10% duty cycle (using a timer to control this) on a normal basis. There is an interesting write up here

In any case, it just makes me wonder if some of the tubehobby recommendations aren't a bit conservative..
Last edit: 6 months 2 weeks ago by Jedidiah.

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