Digit Brightness, Classic Rev6, IN-8 Tubes

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.

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.

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..