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Old 19th August 2011, 07:37 AM   #1
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Default Service Life at Low Plate Current

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 ?
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Old 19th August 2011, 11:02 AM   #2
DF96 is offline DF96  England
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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.
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Old 19th August 2011, 02:11 PM   #3
ryuji is offline ryuji  United States
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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?

Last edited by ryuji; 19th August 2011 at 02:15 PM.
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Old 19th August 2011, 02:49 PM   #4
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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
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Old 19th August 2011, 05:16 PM   #5
DF96 is offline DF96  England
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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.
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Old 19th August 2011, 06:58 PM   #6
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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
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Old 19th August 2011, 07:05 PM   #7
DF96 is offline DF96  England
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Yes. The OP was asking about cathode interface, which can affect ordinary radio and audio valves.
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Old 19th August 2011, 08:09 PM   #8
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Quote:
Originally Posted by richwalters View Post
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
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
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Old 20th August 2011, 01:10 AM   #9
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Quote:
Originally Posted by payloadde View Post
My concern is with low current (plate and cathode), 3mA instead of 18mA.
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?
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Old 20th August 2011, 01:15 AM   #10
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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%20cable.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
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