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.

  • \$\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 '17 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\$ Mar 15 '17 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\$ Mar 15 '17 at 21:53

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

  • \$\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\$ Mar 15 '17 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\$ Mar 15 '17 at 12:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.