VFDs, since you don't want the filament to visibly glow, vary less than other tubes. 2.5:1 that you're getting is rather mild. On typical "radio" tubes, I get 6:1 routinely. And that's on small signal tubes, such as 12AX7s. There, a filament that's ~85 ohms hot, will measure ~15 ohms, cold, on an ohmmeter.
On my 1 Nimo clock, that I made 2 1/2 years ago, the filament supply is pretty simple. The nominal filament is suppose to draw 200mA at 1.1V, for 5.5 ohms, hot. I use an unregulated "12V" supply, which is really ~11V. That goes to the filament by way of 2 22 ohm 2W resistors. The Nimo filament is in series with those in the middle to get the ~6V bias, too. But 2 1N4007 rectifiers are shunted across the filament. See the drawing. Note an extra 1.5 ohms is in series, then that group is shunted in parallel with the two rectifiers, and that whole bunch is in series with the 2 22 ohm power resistors. Its simple and reliable. Even if the cold, the current will only be ~50mA over rated, for the brief time it takes to warm up. Surge currents are usually several times the running current, during warm up. <https://threeneurons.files.wordpress.com/2017/12/nimo_clock.jpg> This clock has only been unplugged for a couple of months, last year, while I was in the process of moving. https://threeneurons.wordpress.com/miscellaneous-projects/ On Wednesday, July 15, 2020 at 6:20:16 AM UTC-7, Jasper C. wrote: > > I'm working on a project using some IV-17 tubes, and I cam across > something I need advice on. The datasheets I've found say that the > filament current is 47 mA at 2.4 Vac, so an effective resistance of about > 51 Ohms. But on 3 tubes I've measured, they're all about 20 Ohms. Does > the filament resistance change when hot? Because that seems like a huge > difference... > -- 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 neonixie-l+unsubscr...@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/47dffc25-1cef-40de-811c-1a4afd8d598co%40googlegroups.com.
