vacuum tube failure modes?

[ftransform]

Diodes can either fail as a resistive short or a open. Which possibility is more likely?

Does a vacuum tube rectifier also have these two failure modes? If it can only fail in the open position then do any electronics use vacuum tubes to decrease damage in exchange for survivability in expensive industrial processes? I figure that they would just add protection diodes but I don't know for sure... especially when high currents are involved.

[SeanB]

They have multiple failure modes. They can get gassy and thus lose control ability. The filament can go open circuit or lose emission so lower current and eventual open circuit. The glass can leak, so first gassy then open circuit. They can get interlectrode shorts and this can be transient or permanent. The electrodes can melt from overcurrent and go open or short.

[PA0PBZ]

I've seen vacuum tube diodes where the filament (used as cathode) failed and shorted to the anode. I think it was an AZ4 (long time ago..)

[jackbob]

It just depends on the case, naturally failing they probably will just put out no power, however like the other user said, some (most of the diode tubes I have seen) have the filament used as a cathode and if this were to break and become loose it could very well short. But because there is a fairly large gap between the two there is no chance of them shorting at all without something physically coming loose and touching.

[dcel]

Mercury vapor rectifiers if new or unused for a long time will be shorted out. Either run the filaments only for 20 min no high voltage or pull them out of circuit and use a tube tester to heat the filaments for 20 min without the high voltage. Tubes will also be subject to flashover, after the high voltage has been on for a while, there can be hot spots in the tube that will arc over,  lower the high voltage level slightly and it wont happen, if it does just replace tube.

Chris

[free_electron]

My preferred method of failure for those things is a big box of hammers ...
i just love the sound they make when you hit them.

[SeanB]

Just remember that all TV, radio and satellite depends on them for power stages..............

[ftransform]

Hmm. Does anyone know of any good books/papers to read about vacuum tube failure modes?
And maybe the modern day uses of vacuum tubes? In 2012.

And has there ever been a tube designed that the physical separation between the electrodes is great enough to make a short failure mode impossible? And I assume the mercury tubes just have a puddle of mercury form on the bottom shorting them out?

[SeanB]

Satellites still use Travelling Wave Tubes as power amplifiers, and they are hard to short out anode to cathode simply by the spacing between them being so big.

see http://en.wikipedia.org/wiki/Travelling_wave_tube

[dcel]

And maybe the modern day uses of vacuum tubes? In 2012.

AM/FM/TV transmitters are tube, industrial RF heaters are tube, guitar amps, microphone preamps, hamradio, audiofoolery, microwave ovens, radar, etc.

And has there ever been a tube designed that the physical separation between the electrodes is great enough to make a short failure mode impossible?

No, due to the high voltage that they operate at and the proximity that the tube components must be at to function. Some older tubes were hardened against shock and vibration but it only goes so far.
 

And I assume the mercury tubes just have a puddle of mercury form on the bottom shorting them out?

Yup! Just heat the heaters only to regasify the mercury and all functions as normal.

Chris

On edit: Check out the very rare and cool tubes that Mike http://www.electricstuff.co.uk/ has on his site. A very nice collection.

[jackbob]

Ah mercury rectifiers are very pretty  they look cool when they are on, used to be used way back in the day ti charge the electric car 1913-ish

[HardBoot]

I've never seen tube diodes of any sort fail shorted, but I have seen much more fancy types of low quality tubes fail shorted.

[vk6zgo]

By far the most common fault with vacuum tube rectifiers is that the cathode will lose emission,so that the internal voltage drop of the device will increase to the point where they become unusable.
Occasionally,you will get a filament go open circuit,but that is a lot rarer,arc overs internally are extremely rare.
Vacuum tube (valve) rectifiers come in both directly heated & indirectly heated forms.

Mercury vapour rectifiers will arc between anode & cathode under certain circumstances,& usually have special protection circuits to prevent this becoming a sustained arc.
One nasty trick mercury vapour rectifiers had was of oscillating,if they haven't been warmed up properly.
Tube rectifiers in Broadcast & commercial equipment are obsolete,replaced by silicon rectifier stacks.Usually,even very old Transmitters were retrofitted with solid state devices.

The latest generation of TV & Broadcast Transmitters are mostly all solid state,whereas the previous generation  was solid state up to the penultimate stage,then tube finals.
The tube final in a VHF TV transmitter is a pretty hard act to follow,with around 13dB of power gain in a 5MHz wide amplifier,20.000 hours average life,easy cooling with a large capacity blower.
A lot of the new solid state finals have gone "back to the future"with water cooling like that used in 1930s tube equipment,& very high power stuff in more recent years.

[T4P]

 guitar amps, microphone preamps, hamradio, audiofoolery, microwave ovens, radar, etc.

(most) Guitars amps and (some) microphone preamps use tubes because the users think it's much better. *as if*
 Microwave ovens? Da hell?

[dcel]

 guitar amps, microphone preamps, hamradio, audiofoolery, microwave ovens, radar, etc.

(most) Guitars amps and (some) microphone preamps use tubes because the users think it's much better. *as if*
 Microwave ovens? Da hell?

The magnetron tube in your microwave is still a vacuum tube!
http://en.wikipedia.org/wiki/Cavity_magnetron

Chris

[T4P]

But it doesn't perform the same sort of tasks . (Rectification and Amplification)

[saturation]

You can find a good list of various failure modes for components on Wikipedia:

http://en.wikipedia.org/wiki/Vaccum_tube#Reliability

http://en.wikipedia.org/wiki/Failure_modes_of_electronics