I am working on an Arduino project involving nixie tubes and I have been stuck with a tube brightness problem for about 10 days. I would greatly appreciate any input on how to solve this issue. I apologize in advance in case I made some dumb mistake, I have some experience with Arduino projects and soldering circuits but I am no expert with electronics.
As i was saying, I am building a nixie clock using eight IN 18 nixie tubes and an Arduino Mega. The tubes are directly driven (no multiplexing, at least for now), each with a K155ID1 driver.
A 12 V power supply is used for both powering an Arduino Mega and as input for a high voltage DC-DC transformer (NCH6100HV). The voltage supply for the anodes ( U_pot ) can be modified through a potentiometer placed on the transformer. In my case I set 170 V before connecting the tubes. A 330 uF capacitor was placed between the 12 V and the ground between the power supply and the transformer. The anodes of the eight tubes are connected in parallel to the high voltage supply, each of the anodes connected through a 10 kohm resistor ( R_an ). The choice of supply voltage and resistors was made based on examples of similar clocks .
After some tests on a breadboard, I have soldered the electronic circuit using a few stripboards. Pretty much all the connections have been soldered and the tubes have been tested and observed to be working correctly: all digits are correctly displayed, the anode current is within the datasheet range  and the tube brightness is appropriate).
These tests were all carried out for a single tube to verify I had correctly soldered a tube socket. However, when I inserted all the eight tubes in the sockets to verify if the clock was working properly, I noticed that the digits were not completely lighted and that the lighted parts were not as bright as when the tubes had been individually lighted. I measured (see Table 1 in the attachments) the current draw out of the transformer ( i_down ) while inserting more tubes in the sockets and noticed that the current per tube ( i_down/N ) decreases from 4.9 mA for one tube to 1 mA for eight tubes. The current upstream the transformer ( i_up ) changes very little.
I tried changing the supply voltage U_pot (see Table 2 in the attachments). However, voltage downstream of the transformer seems to drop at « running » conditions, when current flows through the tubes, to a similar value ( U_run ) no matter the set value of U_pot. Both U_pot and U_run are measured at the HV and GND pins of the transformer.
I read a bit regarding nixie tubes’ physics and, if I understood correctly, this is normal. As long as the voltage is sufficient to lighten the tubes, current is what matters for brightness and voltage will depend on the tube voltage drop and selected resistors.
Since changing the supply voltage was not helpful, I tried changing the resistors R_an. In the beginning 15 komh resistors were used. Decreasing to 10 kohm only slightly changed the current through the tubes, with i_down being 7.8 mA instead of 7.4 mA with eight tubes.
I have read  that undervolting nixies can lead to long delays for the tubes to light up but I do not think this is the case. I tried waiting around ten minutes with four tubes in the sockets and the measured current i_down seems to remain the same.
I would like to have a current between 4 mA and 6 mA in each tube to comply with the datasheet indications but I am out of ideas. Further decreasing the resistance below 10 kohm may help but I do not see why it is not already working when the parameters I am using now correspond to what has been done by other people.
Once again, I would appreciate any help you can provide.
 email@example.com | Undervolting Nixie Tubes
Tables.pdf (23.8 KB)