Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: BJK on March 07, 2021, 06:59:09 AM
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So the 12AU7 looks like it might have a teeny tiny air leak. The chrome treatment stuff that keeps it a vacuum seems almost used up. I’ll post pics here, but for example, I can see the tube light from directly on top. I don’t know that I can remember if it was “always like that,†it’s something I noticed over the last few months.
1- should I proactively replace it? Or just wait till it goes ?
2- what are recommendations for replacing? And where would ya’ll recommend I get it? (Not necessarily interested in tube rolling so much as stock aproxÃmate.).
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I would post a photo. What you're describing does not sound like an air leak.
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Hmm... been trying to upload pics, server seems to keep crashing. Trying a smaller photo this time.
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And here it is from the front:
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I would keep using that tube. It might only have five or ten years of life left in it.
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And here's a shot of the pancakes my wife made, ...for your patience...?
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I would keep using that tube. It might only have five or ten years of life left in it.
Thnx, PB. And for the noobs like me. That would be 5 or 10 out of what? (aka, What's a nice life time to expect?)
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FYI if the vacuum fails and air is getting in the tube you'll see that silver turn white.
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The chrome thing is called getter flash, it is normally made of Barium to absorb residual gas inside the tube.
It seems your tube still has lots of getter left, so no worries. If the chrome is completely gone, or becomes milky white, then the tube will stop working due to losing its vacuum. Until then you can replace it with a new one.
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I would expect an old stock 12AU7 that's lightly used but still testing strong to last 10-20 years with regular daily use.
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How about a 6SN7 using a 6SN7-to-12AU7 adapter? Isn't the 12AU7 running at 6V instead of its native 12V, which is why it lasts so long in the Crack? And isn't the 6SN7 natively only 6V, so it's not running conservatively?
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The 12AU7 filament has two 6-v sections, which are operated in parallel for 6v power, or in series for 12v power. 6SN7 is 6v only, and a 12SN7 is 12v only.
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Ah, so it's not that the 12AU7 is running under voltage. Does that mean the 6SN7 in the Crack should also last 5-10 years?
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If you run the heaters under or over voltage, that is generally detrimental to the tube. A 6SN7 in a Crack should also last a very long time.
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BJK here is a photo of a tube that sucked air. Cloudy grey color, nothing shiny. It sounded fine for a few minutes but as it heated up there was scratching from the channel as the tube began to really glow, the whole thing bright orange as I reached for the off switch.
Karl
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Thanks, Paul! Cheers!
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Super helpful, all. Thank you!
To the quote below. I would say mine doesn't "not look like that." I would describe it as a black-ish (almost chared) effect, with a withering of the chrome. I can post a pic comparing the different tubes. (Same thing is happening to one of the pre-tubes on my stereomor.)
I guess my question now, is what _does_ cause that to happen. Even if it's just normal, curious to understand what's going on in general.
BJK here is a photo of a tube that sucked air. Cloudy grey color, nothing shiny. It sounded fine for a few minutes but as it heated up there was scratching from the channel as the tube began to really glow, the whole thing bright orange as I reached for the off switch.
Karl
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Here's a pic with the two different tubes. You can see the "coloring" of the on eon the right is darker/blacker, and starting to diminish.
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I have pulled brand new tubes out of sealed factory packaging that looked identical to the tube on the right. If this is visually bothersome, you can purchase a new pair of tubes, but there's nothing about that photo that is cause for alarm.
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Nothing wrong with those tubes. Go look at an RCA cleartop, they have a teeny tiny bit of getter material, yet they are one of the nicest 12AU7s.