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Author Topic: Cathode Poisoning  (Read 1721 times)

arconaut

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Cathode Poisoning
« on: February 07, 2006, 11:11:28 AM »

Hey all,

In the "leave it on/turn it off" debates, I've tended to lean toward the "turn it off" side. Partially under the unfluence of Walter Sear's essay -  (http://members.aol.com/searsound/frmcont/onoff.html) - and partially under the premise of not burning out tubes and motors.

But I was reading (I should say "trying to read") "The Art Of Linear Electronics" by John Linsley Hood and I came across a passage that was interesting to me. So at the risk of violating the law, here is a bit of that passage and I'm curious to know your reactions to it. I guess my first question about the passage is does not not really apply to class-A equipment, only to push-pull circuits?

Thoughts?

Noah



"Valve amplifier designers have sometimes advocated the practice of leaving the valve cathodes continuously heated during periods when the amplifier is not in use, partly to avoid the time delay in operation following switch-on from cold, and partly in the hope  that this practice will avoid any damage to the cathode which might arise if it is caused to operate for a short time under depleted space charge conditions during this warm-up period. Unfortunately, leaving a valve at zero anode current, but with its cathode at or near its normal operating temperature, leads to the much more serious possibility of cathode 'poisoning', due to residual gases within the valve envelope combining with the exposed, and highly reactive, cathode surface, to produce chemically stable and elecrically inert compounds - a process which will permanently reduce the cathode emissivity.

In normal use, the flow of electrons from the cathode to the anode will sweep such contaminants away from the cathode, and the normal process of evolution of the uncombined cathode metal by reduction of the oxide at the cathode substrate, followed by its diffusion to the cathode surface, will help preserve a clean and active cathode surface, even in the presence of contaminating gases.

Unfortunately for the idea that leaving the cathode hot will avoid damage due to cathode operation under depleted space-charge conditions, it is pointed out by Brimar, the valve manufacturer, that the electron emissivity of an oxide coated cathode surface falls off much more rapidly with reduced cathode temperature than its chemical reactivity and its proneness to gaseous poisioning."
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bushwick

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Re: Cathode Poisoning
« Reply #1 on: February 07, 2006, 12:12:17 PM »

Correct me if I'm wrong but doesn't the last sentence say electron emmisivity falls off more rapidly because of temperature drop than it does from cathode poisoning.... which is bizarre and changes the conversation from a good vs. bad to bad vs. worse.

Strangely worded to support an argument that turning off gear is better.
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Joshua Kessler
bushwick  studio
brooklyn, ny
www.bushwickstudio.com

John Klett

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Re: Cathode Poisoning
« Reply #2 on: February 07, 2006, 12:20:24 PM »

Okay - I have read similar material...  This is another case of picking out one factor among many so rather than flog away at that (I am tired after thinking too much about diodes just now) how about leaving your tube gear on and running low level signal through it all the time?
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John Klett / Tech Mecca
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