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Old 1st March 2004, 06:11 AM   #1
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Question Tube ageing ?

Would a tube passing 50 v at 150ma age faster than the same tube passing 150v at 50ma? I know that holding the plate voltage constant the higher current will shorten tube life but
is this because of current draw or higher plate disipation?

Thanks,

Woody
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Old 1st March 2004, 06:56 AM   #2
mig-ru is offline mig-ru  Russian Federation
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Current draw. The cathode determines the life of the tube, being coated with exotic metals which give of the "electron vapour," these metals get exhausted over the life of the tube. The plate doesn't play much of a role in tube life unless it is overloaded and melts.
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Old 1st March 2004, 06:59 AM   #3
dhaen is offline dhaen  Europe
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Generally, yes. This assumes it's kept well within it's ratings.
I don't think you can scale this for small-signal valves.
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Old 1st March 2004, 12:27 PM   #4
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Hi,

Quote:
The cathode determines the life of the tube
Indeed it does....
Did you know that you can increase tube life by 50% by reducing heater voltage by 5%?

So, for a 6.3V heater that means setting it at 6.1 V.

The only disadvantage is that it will now take a few extra seconds for the cathode to reach full operating temperature.

Cheers,
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Old 1st March 2004, 12:42 PM   #5
EC8010 is offline EC8010  United Kingdom
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Woody: Your question is rather complex. Ultimately, it's failure of the cathode that determines valve life. However, cathodes can fail due a number of causes:

Tired emissive material.
Poisoned emissive material due to outgassing from hot anode.
Poisoned emissive material due to water vapour released from hot micas.
Poisoned emissive material due to sputtering from nearby electrodes.
Damaged emissive surface due to ion bombardment.
Peeling emissive material due to repeated thermal stress.
Growth of resistive cathode interface layer between cathode conductive sleeve and emissive material.

I'm sure there are lots more, but that's all I can think of for now. However, don't be daunted, I would suggest that the life of any given DIY project is generally shorter than the valves it uses...
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Old 1st March 2004, 02:35 PM   #6
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On a related note, does anybody have experience how high a cathode current for a single section of a typical miniature dual triode such as a 12au7 can you run before impacting tube life, assuming thermal stress and other factors are not a concern? I have an application where I might want to run up to 10mA (at 170Vdc plate to cathode) if that would not significantly impact tube life. The average current and plate dissipation would be not much more than half the maximum ratings in this case.
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Old 1st March 2004, 02:56 PM   #7
EC8010 is offline EC8010  United Kingdom
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According to the Mullard data sheet, the ECC82 (12AU7) is rated for Ik(max) = 20mA, with pulses up to 150mA, provided they are shorter than 200us. 10mA is therefore well within limits and shouldn't shorten life.
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Old 3rd March 2004, 11:38 PM   #8
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Thanks for all the feedback.

Woody
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