This extra "Grid" has no other function but reduce the tubes secondary emission from the filament similar to the suppressor grid in a tetrode tube.
That seems like a good theory. I'm not sure how the extra grid works in that capacity though. I don't see anything between the grids and plates.
Good theory but not really totally true...There are Russian manuals that show the physical construction of the tube but the grid shown on the data sheet i think is just a "theoretical grid"...just showing that there is some control of tube gain to keep it from oscillating...I have seen a few tubes actually go into oscillation...strange but true!
If you find an IV-18 that has a higher than design gain, try putting your hand near the glass envelope and you will see the digits get brighter.
Neat idea. I'll have to try that!
They all do not do that...I would say less than 10 percent do...
look up the term "Filament Notching" ... could you tell me the difference between the heating or the thermionic effect of 5 volts AC RMS and 5 Volts DC?
See report from the Air force Wright Patterson A.B. WRDC-TR-90-4075...just one example but there are many of the effect of D.C. both on vacuum tube filament and incandescent lamps. It is the chemistry of the filament wire that determines how bad it can be and testing will show if they are degraded. These In-18 filaments seem to be quite strong but who knows?....only the engineer that designed the tubes...I will try to see if he is still alive...probably not but i am sure there is documentation.
I'm struggling to understand the point here. If you're claiming that, for some unforeseen reason, driving the filament with DC might reduce tube life, then I completely agree. But if you're claiming that driving the filament with DC is likely to reduce tube life, then I remain skeptical.
One example see report WRDC-TR-90-4075
As I understand it, filament notching is mainly a problem at high current densities, but the filament on the IV-18 barely glows. In regard to thermionic effects, I believe electromigration is also a problem only at high current densities. Filament voltage remains well below the maximum negative cathode to grid voltage, so cathode poisoning due to plating seems unlikely.
Unknown filament chemistry but perhaps you are correct....
I think the main concern for VFDs is phosphor life, and the choice of AC or DC on the filament should not affect phosphor life so long as brightness is consistent.
Yes they will get dimmer over time...some designs DO step up filament voltage to deal with the fading...that is why they are run at minimum operating parameters at first to prolong phosphor life....
The hahaha effect on your clock
Just a pun....that was an experiment on a flat display...the filament was tested under direct current...the voltages were measured at intervals along the length of the filament and as you can see they started at zero at one end and ended up at source voltage at the other end....this was done on a test display filament under vacuum with taps along the length of the filament..fairly linear voltage drop from one end of the filament to the opposite end...
I'm also puzzled by the purpose of the brightness gradient photo. The caption is "The hahaha effect on your clock," but the photo is neither an Ice Tube Clock nor even an IV-18. I don't understand the relevance.
I'll conclude with the following quote from another discussion
We use VFs on several of our products, all with DC filament drive. Maybe 10,000 or so by now, some in the field for 15-20 years now, usually running 24/7. I don't know of any filament failures. DC seems to work fine.
Yes.... some automotive applications use DC but again they are not Russian IV-18's...lol...
The filaments run at much lower tempearture than light bulbs, and light bulbs that run dull red have enormous expected lifetimes.
All things considered, replacing R3 with a jumper still seems like an acceptable solution for a DIY bedroom clock.