PhilD13 wrote:The LM9022 is no longer available. You can take a look at the MAX628, also known as the TSC428 to generate the filament drive.
Performance of the related MAX627 (on my breadboard) is comparable to the FET push-pulls. But the FETs are cheaper and require less board space than the DIP version of the MAX627, so I'm thinking that I want to stick with the FETs.
Russell 27 wrote:Raid's in grid IV-18 He claimed [filament drive] was not so important, in his opinion.
Raid was driving the IV-18 filament well below the 5 volts required by the tube specifications--around 3.5 volts DC. At that voltage display gradient is barely noticeable.
As I understand it, the predominant failure mode for vacuum tube filaments is cathode poisoning, which is greatly accelerated by running the filament outside of the specifications. If voltage is too low, the cathode is too cold and emits fewer electrons. The large positive charge at the anode and grid ionizes trace amounts of gas in the tube, forming positive ions. These positive ions are attracted to the cathode, but are usually neutralized by electrons. If the cathode is emitting fewer electrons, a larger proportion of the positive ions collide with the filament, damaging the oxide layer and resulting in reduced electron emission. Driving the filament at too great a voltage accelerates sublimation of the oxide layer--the other common cause of reduced electron emission--but running a filament too hot is not nearly as damaging as running it too cold.
The effect of cathode poisoning is readily apparent in the first post of this thread
. Note how the time displays nothing on the 3rd and 6th digits. When the time is displayed and the cathode is too cold, the protective electron cloud surrounding the cathode moves towards the more positive regions--the lit digits--and that depletes the protective cloud surrounding the filament near the blank digits. Positive ions collide with the filament's oxide layer near the blank digits, resulting in reduced electron emission. In the clock menus, the 3rd and 6th digits are significantly dimmer than the other digits because the cathode has been disproportionally poisoned in those regions. Running the filament to spec--5 volts--should fix that.
Unfortunately, running the filament with 5 volts DC causes a slight gradient across the display, but that gradient becomes quite noticeable if brightness is lowered by reducing the anode and grid voltage. EDIT:
And DC is associated with filament notching, although I don't think notching would be a problem given the relatively low current density in VFD filaments.
I hope that explains why I'd like to run the filament at 5 volts with some form of AC.
Russell 27 wrote:Try driving your MOSFETs with a 9 volt gate potential and see if that helps, 4.7 volts from the micro controller is marginal. Or try boosting voltage from LM317 a little, with variable resistor. Adjust variable resistor while reading filament output from oscilloscope, see if you can cheat voltage up a little. Using some capacitance should clean up your wave.
Surprisingly, those MOSFETs are already in saturation at 4.7 volts, according to the datasheet! The scale on voltage for the oscilloscope pics is 2 volts, so the wave is going -5 volts to +5 volts--just where I want it to provide 5 volts of square wave AC.
Russell 27 wrote:Using some capacitance should clean up your wave.
Yes, it does. But board space is tight, and I'd rather not squeeze in another cap, and if I add a capacitor, I'd also want to add a protection diode. My thinking is that if sine wave and square wave AC are both okay for the filament, then a slightly noisy square wave should be fine--as long as the magnitude of the wave never exceeds 1.4x the RMS voltage.
So here is my question: Do I need to clean up the wave? The filament should be insensitive to the exact shape of the wave, correct?
For reference I'm attaching a draft
of the xmas-icetube Rev. D schematic and board and some images.