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First some context: I am considering building a decoration out of a old RF power vacuum tube (like a Eimec 3-400Z, Telefunken RS630,...) with direct heated tungsten filament. I plan to get a appropriate SMPS to power only the filament for a nice glow in the living room & already know what to look out for as to not burn down things (already built a tube audio amp from scratch).

Since I don't want to actually drive the tube with a signal, no high voltage is necessary (plus I wouldn't put 3kV into my living room). This also means I likely don't need to worry about heat, e.g. the 3-400Z would have a plate dissipation of typically 400W when used in a power amplifier.

My question: I found that there are claims of tubes decaying faster if only the filament is heated an excessive amount of time; I wonder if this will cause the "light" to break quickly, or if it doesn't matter since it will only degrade the characteristics (German "Kennlinie")?

I found the similar question I need a basic primer on interpreting vacuum tube datasheets - but even if I only would turn the "tube light" on for decoration a few hours per week, I suppose I will operate it way longer than they shot that movie.

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  • \$\begingroup\$ When it is cold, the filament has a very low impedance that creates a big current rush when switching on. This is one cause of breakdown. Add a 33 ohms NCT inrush in your power supply to limit this current. \$\endgroup\$
    – greg
    Mar 15, 2017 at 9:13
  • \$\begingroup\$ Would a bright orange LED next to the glass at the base of the tube work to give a nice glow? \$\endgroup\$
    – Andrew Morton
    Mar 15, 2017 at 15:20
  • \$\begingroup\$ @AndrewMorton For smaller tubes that works, but I am looking at a larger RF power amp model in the kW class. I am eyeing one on fleabay which is rated with >150W - for the heater. I believe for those "cheating" with a LED becomes difficult/obvious. But since the tube will become a smart light, maybe I put some Neopixels to good use below the base? \$\endgroup\$
    – ArchimedesMP
    Mar 15, 2017 at 21:53

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Perhaps they're talking about indirect-heated cathode-oxide degrading? See what they say about the failure mechanism.

In common audio/radio tubes, "degrading" means the oxide gets contaminated, not that the filament breaks. Yellow-glowing, direct-heated filaments are not commonly encountered in tube circuitry.

In any case, if you run a tungsten filament at 99% of rated voltage (or 95%, or 90%,) the expected lifetime becomes huuuuuuuge! Simple: un-tape the filament transformer winding and pull off a couple feet of secondary wire.

Heh, after I'd done that just once, I realized that transformers ...they have user-servicable parts inside! You'll never leave your transformers alone any more. Also you can dip transformers in polyester resin or hot wax to greatly increase isolation voltage of floating windings (even pull a temporary vacuum to clear out any air bubbles. Who says that vacuum-impregnation was complicated?!) Also, 'tune' your 2watt carbon resistors with a triangle file.

Also Vaporizing cities with radio, 1947, no tubes in sight! "Krakatit," Karel Capek

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  • \$\begingroup\$ Ah, yes, the tubes I am considering are all tungsten filaments (70 - 170W power draw), good point. Nice hack for the transformer, but I think I'll just play with the voltage adjust of the switching PSU. Plus I think I'll add PWM for dimming (after heating phase). \$\endgroup\$
    – ArchimedesMP
    Mar 15, 2017 at 12:09
  • \$\begingroup\$ They say When powering a tube with a 'alkaline earth oxide' coated cathode (which is the case for all "modern" tubes) for a very long time without anode voltage and therefore without anode/cathode current, a 'intermediate layer' reducing the emission will build up. (yes, that's one sentence - welcome to German). So that's basically what you say, I just did not understand it properly until I translated it from my native language to English :) \$\endgroup\$
    – ArchimedesMP
    Mar 15, 2017 at 12:16
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You could try heating the filament with a PWM generator. This way the filament may last longer since the current will run through it only a small fraction of time, not continuously. The voltage can be increased a bit to still achieve normal brightness yet have the current interrupted.

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    \$\begingroup\$ I'm afraid this is a poor answer, as the lifetime of a filament is determined by its temperature, not the current flow pattern; at least until such extreme peak currents where electromigration might be an issue, I suppose, which AFAIK doesn't apply to tungsten filaments at AC/DC. But that's a potential mechanism where PWM would reduce life, even, not increase it. \$\endgroup\$
    – Tim Williams
    Sep 4 at 17:39
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Treat the filament exactly as if it was a light bulb - but a long life one. You can power it just enough so that it glows as desired. Everything else you've read is about the performance of the tube as an amplifier, not as a light bulb. In the "light bulb mode", vacuum tubes are extreme performers, as long as you soft-start them.

There's no problem with running the filament at the rated voltage or a lower voltage - per your esthetic criteria. And that's pretty much all there's to say about it, and that's a good thing :)

There are various soft-start circuits out there. Perhaps the simplest one is a series resistor with a delay-timed relay bypassing it. Otherwise, a SMPS configured as a constant-current sourcew ith voltage limit set to the rated filament voltage will be just about the kindest way one can treat the filament.

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