Not just a good reply, an exceptionally well thought out and executed reply.
Mike, K6BR
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of david sutton
John.
Phew.
Good reply, just what i like to see.
Dave
KG4UXR
________________________________
From: John Lyles <jtml@losalamos.com>
To: amps@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@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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