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Old 17th July 2011, 06:33 AM   #1
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Default Back From the Dead- A NOS Tube Story

Before you throw that 'bad' tube away, read this.....

A couple of days ago a sleeve of NOS RCA 12AT7s arrived in the mail. Yesterday I pulled out the tester (Eico667) and tested a bunch, including the NOS ones.
I was surprised to find that one of the NOS RCAs tested 55/74. (On the 667,<60=fail, 60-80=??).
This morning I was talking to John Albion (PacificTV-well known tube dealer) and asked him if this (NOS testing bad) happened very often.
Answer:"Sure"
Then he continued, telling me to put the bad tube to work for a few hours and test it again..that often the tube will come up to spec.
Back home, I put it in the RH84 amp, played a CD, and tested again. Result: 80/88
Back into the RH84 for another couple of hours....120/100.

Obviously this won't work for shorted tubes- you don't want them in your amp - but for low emission, it's worth a shot, IMO.
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Old 17th July 2011, 11:50 AM   #2
SY is offline SY  United States
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Sometimes, this can work by re-activating the getter.
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Old 18th July 2011, 11:25 AM   #3
DF96 is offline DF96  England
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Surely reactivating the getter will reduce positive grid current by mopping up gas but won't increase emission? More likely that the cathode was reactivated, although how it got deactivated in an unused valve is unclear.
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Old 18th July 2011, 12:57 PM   #4
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Any chance its the getter material itself which gets driven off the filament ?

Thanks
-Antonio
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Old 18th July 2011, 01:03 PM   #5
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Ive had something similar....

Running the heaters at 20% over rated for about 1/2 hour normally does the trick....

This can also help with some marginal used valves as well...
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Old 18th July 2011, 01:47 PM   #6
DF96 is offline DF96  England
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When an oxide cathode is first made it has poor emission. At the factory, after the vacuum is put in, the heater is run at well above its normal power for a period (minutes, hours?) to activate the cathode. This does something to the surface (metal ions migrate to there?) which makes it emissive.

Sometimes an apparently worn-out cathode can be reactivated. I'm not sure why an unused valve would need reactivating, though.

Getters often work better at higher temperatures. Some only work at high temperatures, although these are generally only used in high power valves. Reducing gas is unlikely to improve emission, although it may reduce the rate of decline in emission by protecting the cathode from positive ion bombardment. Less gas should mean less positive grid current.
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Old 18th July 2011, 01:50 PM   #7
Pano is offline Pano  United States
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Quote:
Originally Posted by DF96 View Post
...after the vacuum is put in...
That's an interesting way to phrase it.
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Old 18th July 2011, 01:56 PM   #8
DF96 is offline DF96  England
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I bet you are a blue-smoke denier!!
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