[Amps] Dimished Tube Life versus emission

[david sutton]

John.

Phew.

Good reply, just what i like to see.
Dave
KG4UXR
 




________________________________
From: John Lyles <jtml at losalamos.com>
To: amps at contesting.com
Sent: Friday, September 25, 2009 12:32:29 PM
Subject: [Amps] Dimished Tube Life versus emission

This reply is based on my own experience, plus talking to various tube manufacturers over the years, dealing with larger power tubes. Emission lifetime, of course, depends a lot on how the tube was set up, was the filament burning at the rated voltage or turned back to preserve life? Also on how much gas evolved in the tube during operation. This could be either from the original manufacturing, or from overloading the tube elements at high average power. Stray ions can damage the emitter and reduce its life. 

Lets stick with one type of emitter for this answer, thoriated tungsten (TT). This means this discussion doesn't necessarily apply to 8877/3CX1500A7, 4CX250A, and those sorts of tubes having oxide cathodes. Someone else can reply about those:
With TT, the filament is carburized with a layer of carbide during processing, to greatly enhance emission from a single molecule-thick thoria layer in the filament.  This coating, plus the thorium percentage in the tungsten, are factors that give long lifetime. If the tube becomes decarbed, where the carburization is depleted, then it will promptly run out of this level of emission that is required to support the desired cathode current. Actually the emission would just fall off a tremendous amount. The only way to recover it would be to raise the temperature much hotter, which will physically destroy the filament. So it is a loosing game at this point. Usually it is not the depletion of thorium that causes the emission shortfall. 

When emission starts to droop, the usual solution to squeeze a little bit longer life out of it is to raise the fil. voltage. This, indeed will improve things for a short time. Then it slumps off again, and the voltage is again raised, and so forth. Usually its a matter of weeks to months for a large tube operating 24/7 before it cannot be increased anymore, the filament setting is too high, power supply ran out of range, or the filament breaks. This also becomes  a nuisance, having to continually tweak the voltage. If you just turned it up, say 10%, you would see a burst of emission, but then the lifetime would be reduced greatly in terms of time duration before failure. Don't do that. 

Now this brings up a question of "what does 75% output mean". In a particular circuit, the designer could choose to use an oversize tube, that has higher emission and cathode current than is needed for the circuit. This would be a costly practice, but is sometimes used in pulsed power systems (such as where I am at work) where a big tube with lots of plate dissipation is used, not for the plate dissipation, but for the higher cathode current (and plate current as noted on characteristic curves from the datasheet) during the pulses. In this case, extreme long life can be had by reducing the filament voltage a bit,  on the order of years. A used tube pulled from this application may have a lot of life left in a lower power system. On the other hand, if a particular circuit designer chose a tube close to its ratings, for peak current and for dissipation, the filament may need to be near its nominal rating, which means the lifetime will usually be somewhere
 on the average like 10
,000
hours or less. In this case, when the tube performance degrades to 75% power, then there isn't much left to go on. The tube will rapidly decline to the point where 50% or less is all it may do. So this question of 75% output, really reflects on how the original circuit was running this tube. A pull from an AM broadcast transmitter modulator may be really spent, as they run them until distortion creeps up or cannot reach 100% AM anymore. 

There won't usually be a short that ends life for a decently designed tube and circuit. It will typically be fall off of emission until the peak plate current can not longer be reached at the peaks of the RF or audio waveforms. If a TT tube is run for many years, however, the TT filament does become aged and brittle and is subject to breaking if the tube is handled roughly while the filament is cold. Another thing that may indeed cause a short is to try and raise the screen voltage for a tetrode, or drive a grid harder for a triode, to try and compensate for reduced output due to emission decline. This can cause an overheated element which may indeed short to another element and kill the tube. 

This reply is not straightforward, but neither is end-of-life for a tube. A lot depends on the circuit, how the tube was handled, how close to rated current, filament voltage regulation, etc. When a tube starts to fall off in power, and it is closed rated for the circuit design, then it will continue to fall off for some time. If this is not the case, then you should be able to bump up the filament voltage to get more output, at least for a while. There is no single answer that covers all tubes with your question, a lot depending on the circuit and the conservatism of the engineer who designed it and selected a particular tube. You can continue to use it to zero output, but the decline will be fast at this point, as it will inevitably become de-carburized. Good luck with that. 

73
john 
K5PRO


> Message: 7
> Date: Fri, 25 Sep 2009 11:05:38 +0000 (UTC)
> From: Paul Decker <kg7hf at comcast.net>
> Subject: [Amps] Dimished Tube Life
...
> When is a tube considered at it's end of life??? Of course a tube like I posted about last week, where there is a grid - cathode short, is (at least right now) at its end of life.?? But if over time the tube is simply having reduced output or "going soft", at what point is it considered "no good"; at 95%, 80% 75% output????? 
>
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