Hi Folks,
I know that tubes deteriorate when run at Zero plate current for extended periods of time.
But what happens at low - non zero - currents.
In my application a tube rated at 40 mA abs.max and 18 mA typical will actually be run at just 3 mA.
Will this reduce service life similar to zero mA operation ?
I know that tubes deteriorate when run at Zero plate current for extended periods of time.
But what happens at low - non zero - currents.
In my application a tube rated at 40 mA abs.max and 18 mA typical will actually be run at just 3 mA.
Will this reduce service life similar to zero mA operation ?
I'm not sure. Do some searching and reading on cathode interface layers (also known as sleeping sickness). My guess is that you may be OK, or you may have a slightly reduced service life but not too much to worry about. Alternatively, find a special quality valve with a cathode designed not to do this.
are there any examples of commercial class B tube amplifiers? that'd probably be your best indicator. I suppose any practical tube circuit will just be an class AB case, or borderline AB/B as the original poster seems to be inferring?
from my understanding cathode poisoning happens from the space charge collecting far too large, and you simply need any sort of voltage gradient between the two plates to prevent it? some diy circuits i have seen use a resistor to drop b+ real low to effectively 'bias the amplifier real cold' to attempt to avoid this?
from my understanding cathode poisoning happens from the space charge collecting far too large, and you simply need any sort of voltage gradient between the two plates to prevent it? some diy circuits i have seen use a resistor to drop b+ real low to effectively 'bias the amplifier real cold' to attempt to avoid this?
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It's generally assumed varing heater voltages is far more damaging to tube life than the anode exposed to min or overvoltage operation.
There was an article on this very issue which appeared in magazine "Sound Practices", Fall 1994 by Prof. A.V.J Martin, "Factors determining tube life". It's copyrighted so unfortunately I can't image it.
richy
There was an article on this very issue which appeared in magazine "Sound Practices", Fall 1994 by Prof. A.V.J Martin, "Factors determining tube life". It's copyrighted so unfortunately I can't image it.
richy
Not cathode poisoning, cathode interface. Two quite different effects, with two quite different causes. Cathode interface is the slow build-up of a resistive layer between the cathode metal and the oxide coating, when the valve is hot but taking no cathode current. Cathode poisoning is damage to the surface of the oxide from ion bombardment when there is no space charge due to running too much cathode current for the cathode temperature.
Aye--But Cathode-Poisoning doesnt occur with signal and your average O/P bottle...
It only occurs with tubes running above about a kilovolt plate or so...ie--Transmitting tubes
It only occurs with tubes running above about a kilovolt plate or so...ie--Transmitting tubes
Yes. The OP was asking about cathode interface, which can affect ordinary radio and audio valves.
Plate voltage is actually nominal in my case, 100 V plate with 200 V supply.
Heating is nominal, too.
My concern is with low current (plate and cathode), 3mA instead of 18mA.
All literature I found so far talks about cathode interface buildup under complete cut-off conditions (heater on, plate / cathode current zero). And with underheating.
But there is no indication what might happen between zero and nominal cathode current.
Is 1mA safe or 1 uA, 1% or 10% or 50% of data sheet recommendation ?
examples:
Cathode Interface
The Tube CAD Journal,SRPP Optimal Rak Value 4
I've go no scientific basis for my assumption, but I'd wager that in 30 years you might be able to come back and give us the answer. I'd expect that at low, but non-zero plate currents you should experience very long valve life. Perhaps the heater will burn out before the cathode becomes non-emissive.
Can you tell us the specific tube you intend for this application?
Valves used in submarine cable repeaters were purposefully operated with relatively low cathode current density compared to normal tubes (http://dalmura.com.au/projects/Submarine cable.pdf) - which may indicate that your design life may increase (all other things being optimal). If you can locate a datasheet for such a high-rel tube then it may be more informative.
Ciao, Tim
Ciao, Tim
There are plenty of valves that were always intended to run at sub-1mA current levels, and the design of their cathodes is unlikely to be different from any other types. I certainly wouldn't expect you to see problems at 3mA. Maybe 3uA...
The repeater valves in the submarine application were special (read: expensive) versions fabbed with ultra-pure cathode bases.
These were needed because this Interface Layer Formation (Zwischenschichtbildung) is the outcome of a solid-state reaction between elements of the oxide layer, and impurities in the nickel base.
Valves were later developed with this problem minimised by using powder metallurgy to form highly controlled composition of the raw material. It appears that forming the usual tubular sleeves was not possible in this case, since sheet materials were used in such SQ valves - or at least those made by Mullard. At the Blackburn factory these were known as the "Lock-Seam" cathode, the making of which could be seen wondrously in the Mullard video, hosted by Techtube until their demise.
As Tim's referenced article shows, Interface Layer forms even if normal cathode current flows. It's just faster-forming if no anode current flows, which may give rise to speculation that the solid state reaction which yields the layer is retarded by a galvanic effect.
Remember, it's a very long term development, so even if the type you are using has an ordinary sleeved cathode, something else will probably fail first.
On the other hand, Interface kills the cathode if the added resistance in the cathode circuit spoils the design of the stage. If a degradation in gm of 10% will ruin your amplifier, and it is running all day, Interface may be the lifetime limiter.
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I would say low beam current does not wear the tube more or less.
maybe long term bad storage can oxidize the tube cathodes.
what does effect the tube life/performance is electrode polarization ( either ionic or electromagnetic ) because that effects the electron beam as to how strong the power transfer to be and more/less chaotic of an electron beam.-
maybe long term bad storage can oxidize the tube cathodes.
what does effect the tube life/performance is electrode polarization ( either ionic or electromagnetic ) because that effects the electron beam as to how strong the power transfer to be and more/less chaotic of an electron beam.-
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How can a cathode sitting in a vacuum get oxidised? If the valve leaks then it stops working anyway.DavesNotHere said:
Electrode polarisation? What is that? I have no idea what your last sentence means.
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