I talked to actual engineer yesterday, which is of course more experienced than me, and we encountered a problem when putting the filaments in series. If we put, say, 4 filament in series, the upper filament is at 4.5v over GND (if we don’t put any offset). In these tubes the filament is also the cathode which means that the Anode-cathode voltage drops the nearer you go to the top filament. This obviously results in a brightness decrease on that tube. So the optimal arrangement is to put them in parallel, so that they have all the same voltage to GND.
Has anyone ever noticed anything when putting filaments in series?
Speaking of positive filament side.
I have this picture that is taken from the datasheet of the IV-12 I have translated some months ago
It shows little stripes on one of the filament contacts. On the pin number list (up), the document also translated to that “conductive layer on the inner surface of the tube”. This might pose some question about the polarity of the filament voltage. Someone might come across this and answer, it will add to our collective knowledge, or I’m just dumb and everyone knows it already and I don’t
Please see back to my earlier comment about multiplexed or non-multiplexed. If you are driving the tubes from 25V then the 5V drop is significant. I would not recommend putting them in series for your case.
No worries, 64bittz! This is turning into a good thread for future reference.
Regarding the "conductive layer..." on the IV-12, I have no idea what that is about. Are they turning the entire inside of the glass envelope into a cathode?! If I find out anything about it I'll post it here.
Look into it later when the dust is clearing off the crater.