Transforming 7-segment LED clock into IV-6 VFD clock

Take a cheap 4-digit 7-segment LED clock kit (like Bangood SKU142210, about 6.5 USD [red, without case]), design an adapter board and replace the display with four IV-6 VFD Russian tubes. That's what I have been working on during the last three weeks.

According to the schematic, the original clock uses common anode displays and multiplexes all four digits: IV-6 VFD satisfy this requisite, even though I have no idea how fast the multiplex is and whether tubes can react that fast.

IV-6 on veroboard, note the 3rd tube leads.

These VFDs require a grid and anode drive of 12-30 V or more, while LED displays and the clock run at 5 V. After few tests I opted to keep the 5 V input voltage (ubiquitous USB...) and insert a step-up module to obtain 12 to 35 V, which also controls luminosity.

The clock microprocessor outputs a low logic level to turn on segments, while VFD requires a high "logic" level, so the adapter board must both adapt voltage levels and invert the signal. Well, the ULN2003 darlington transistor array is fit for this purpose. Since I need to control 7 (segments) + 1 (digital point) + 4 (tubes) I need 12 lines, two ULN2003 chips (total of 7 + 7 = 14 transistors).

For the sake of simplicity I opted for wiring in series the four VFD filaments that operate at 1 Vmax and add a voltage-drop/current-limting resistor on the cold end: this ensures that the anode voltage is below the grid potential, so the segment turns completely off (otherwise it could still be visible in complete darkness).

Wiring up the boards requires a lot of concentration.

In order to limit the current through ULN2003 transistors when they are "ON", meaning a segment is "OFF", I needed to choose a suitable pull-up resistor value: too high and the current will not be enough to switch off, too low and the overall current consumption increases as well as unnecessary heat dissipation. 3k3 ohm is fine but pretty low, 9 mA apparently not worry much, but they mean 270 mW if I run anodes and grids at 30 V, on a single resistor. 51 kohm with their 51 mW (@ 30V) are too much and Darlingtons don't turn off properly. At least at a first test, but I want to add decoupling capacitors on the high-voltage side because moving wires around seem to fix the problem.