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In Reply to: RE: Tubes need "re-burn in"? posted by pc123v on August 02, 2017 at 12:47:32
I've observed many time that a tube that hasn't been used in a long time (10 years or more?) sometimes need to be fired up (heater only is ok) for some period of time (20 minutes to an hour) to "remember" that they are tubes.
I have no idea what the mechanism is, but I've seen it often enough to now that it's not a fluke. This happens maybe 30 percent of the time on tubes that "think" initially that they're weak.
I don't think that two years would be enough to matter.
At any rate, it's certainly worth the heater time on an otherwise nice tube.
Follow Ups:
Lee, I read someplace years ago one theory that said it's cathode contamination that initially interferes with radiation. After things get good and hot the contamination burns off and bingo. It sounds plausible.
You and Lee are both on the right track...
1. Internal tube parts can continue to outgas while the tube is just sitting in storage. If the getter is in decent shape it can absorb the gas when the tube gets hot - but until it does the tube may not behave itself. It's best to run it a while on a test rig or ?? if possible before putting it into service.
2. Cathodes can become contaminated ("poisoned") and until/unless the contamination is gone the tube's cathode performance is compromised.
Be careful, especially with used power tubes! I recently got a SMALL batch of NOS Ei KT-90 type II that had been on the shelf for years and 4 of the 6 tubes had high gas levels which could have resulted in them running away in an amp. I ran them in my match/test rig for an hour each and the gas was gone - the tubes were perfect.
I hope this explanation helped!
Jim, regarding your question marks, would the application of proper filament voltage do anything, or will the tubes need to see B+ type voltage to "reform"? (for want of a better word)
"Suddenly, I'm not half the man I used to be. 'Cause now I'm an amputee" J. Lennon
Running it with just the heater will help heat the tube so the getter can work to absorb gas; but it may not help if the cathode is poisoned. However - if the tube isn't gassy the chances of it running away are slim. So you could put that tube in an amp and run signal/music through it to see if the cathode comes back with very little risk.
Some guys like to literally bake used tubes in an oven before testing or using them. Noted tube audio book author Morgan Jones suggests this procedure:
1/ Put cold tube(s) in a COLD oven, and set the thermostat to 150 degrees C/300 F.
2/ Once the oven is up to temp, bake the tubes for FOUR hours at this temp.
3/ Turn the oven off and go to bed.
4/ When a-b-s-o-l-u-t-e-l-y cold, take the tubes out and they should be like new.
I have not used this method but many have with success.
I hope this helped.
I recall reading about guys baking old tubes* before, but don't recall if I'd ever actually seen the recipe.
My only question is regarding step number three:
" 3/ Turn the oven off and go to bed. "
Why do I have to go to bed, and what do I have to do when I get there?
No, just kidding.
Thanks for posting the directions, Jim. I've already copied and pasted them into a file for future use. Perhaps now I can reconsider those ebay 6P3S-E which I still need a few of, since you no longer carry them, and try the Morgan Jones Pillsbury bake-off method to de-gas them.
*NOS carbon comp resistors too
"Suddenly, I'm not half the man I used to be. 'Cause now I'm an amputee" J. Lennon
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