VFD heater question

Hi Ian,
a dim glow is absolutely normal. I have various VFD displays in use (video recorder, equalizer, amplifier, and other HiFi equipment, Linux VDR, and last but not least some VFD clocks). In every of these displays I can see the filaments glowing - I do not possess a single device with VFD where the glowing of the filament is not visible. Normally modern VFDs are very bright and you have some dark grey glass in front of the display so that the glow would be only visible when the ambient light is quite dark, so this is normal.

But to my experience a high temperature of the filaments will accelerate the aging of the tubes. So for my clock I reduced the heating voltage below the nominal voltage given in the datasheet.

But another point: Did I understand you right that you connected the three filaments in series? That will not work. The biasing of the filaments is quite delicate, many tubes must be driven with AC because the small voltage drop across the filament will cause uneven lighting. If the bias voltage is too low the segments may be turned off completely while if it is too high neighboured unlit segments may cause fading/blanking effects of lightened segments.

One example: The colon tubes in my clock, the IV-15, need 50V anode voltage and 0.6V heating voltage. As the DT1704 use 1.2V..1.6V, I reduced the voltage by a series resistor. The position of this resistor (GND or VCC side) already has a clearly visible effect, and we talk about 0.6V..1V change in the bias voltage.

With best regards,
Torsten