VFD grid current. Any ideas why it's so high ?

[gregebert]

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.

[John Rehwinkel]

Yup, vacuum tubes (and VFDs, which are essentially triodes) draw grid current when the grid goes positive. Remember the "space charge grid" tubes designed to work with 12V on the plate? The first "space charge" grid would draw a few hundred mA to support a dozen or so mA of plate current. However, wasting huge amounts of current is no big deal in a vehicle.

VFDs are the same way, the grids eat up a lot of current.

- John

[Tomasz Kowalczyk]

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!

[gregebert]

On Friday, January 25, 2019 at 3:27:25 PM UTC-8, Tomasz Kowalczyk wrote:

> 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.

I noticed the pattern of lighter hair-lines on the phosphor-coated segment anodes would change as the grid voltage was varied; perhaps there is a tradeoff between uniform segment glow and how much energy is wasted thru the grid. 

[gregebert]

I peeked at some ILC1-1/7 listed on Ebay, and there was a photo with a tape measure showing "14" .

My heart skipped a few beats, then I realized it was centimeters, not inches. Still, these things are gigantic.

They would make an interesting "see-thru" clock.

[Tomasz Kowalczyk]

Pros of using metric system - if I made such mistake, I would be positively surprised!
I don't know what to do with them yet. I've purchased them because they were cheap and could be gone any second, not because I needed them for any current project :) 
For a see-through clock I'm afraid that I won't be able to attach them securely - the evacuation nipple is on the back of those tubes and looks like it's easily damaged. Also, as those are flat displays, I think they wouldn't look good in traditional nixie tube clock style. Maybe with a bottom and top casing, but still the evacuation pipe would be exposed.