Yes, this high grid current is normal. Standard vacuum tubes, which work as amplifiers, not displays, have different construction in which grid is very close to the cathode, and anode is also relatively close to the grid. This allows the tube to operate with negative grid voltages, at which grid current is so low, that it is usually omitted in calculations. Display tubes, on the other hand, have fairly large - and what's more important, uneven - distance between electrodes. I think that the highly positive grid voltage allows to make electron "beam" hit all anode areas more evenly. If I recall correctly, in some older VFDs you could see that anodes glow slightly brighter under filament wires.
The other advantage of using highly positive grid bias is that you can use the same voltage rail and driver type for anodes and the grids. It can also be way lower than traditional vacuum tube supply (going well over 100 volts). I think that you could experiment with decreasing grid voltage and increasing anode voltage - as long as anode power dissipation stays the same and grid power dissipation doesn't go wild, it should work perfectly fine. It might be a good idea to grab some less valuable VFD display, for example some scrap one and torture it with low grid voltages and high anode voltages. Some people experimented with using VFD (modern, relatively low voltage ones) as normal triodes and succeeded with lower grid voltages and higher anode voltages. http://www.hpfriedrichs.com/radioroom/vfd/rr-vfd.htm BTW - currently waiting for my ILC1-1/7 set to arrive. Can't wait to see how bright those will be! W dniu piątek, 25 stycznia 2019 23:23:34 UTC+1 użytkownik gregebert napisał: > > I just received some ILC1-1/8 VFD's (large-ish 7 segment Soviet era, > approx 100mm tall) and did some basic testing. > > I was very surprised to see a large current on the grid, on the order of > 30-40mA, whereas the segment current was around 5mA when glowing nicely. > I havn't dabbled with vacuum tubes since my high school electronics class > in the 1970's, but I always remember the grid current was very small (of > course....if it wasn't small the tube isn't doing it's job as an > amplifier....) > > Anyways, for any of you VFD folks out there who have experimented with VFD > tubes that have a grid, were you seeing significant current ? > I havn't found the official datasheet, but the info I gathered does state > there is a grid current in the 100mA range at higher voltage (around +35V). > My own experimenting found that I get good illumination at lower grid > voltages (which means lower grid current) around +18V . > > I thought that perhaps the grid + filament is operating like a rectifier > tube, where the grid is acting as an anode. Normally, the grid is biased > negatively to shut-down the tube-current; with this particular VFD the grid > is positive. I have confirmed that positive grid bias is needed in order to > get the tube segments to light-up. I also confirmed that around +12V of > grid voltage, the current starts to ramp-up. > > I'm reluctant to drive the segments at a higher voltage and leave the grid > at a low-enough bias such that it's current is zero; in the end it's > probably a zero-sum for reducing power dissipation. > > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/8f910c5d-c8d0-436c-b26c-b1a6083d785d%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
