CF 275 KT 120 tubes in a fog

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Don
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CF 275 KT 120 tubes in a fog

Post by Don » Sun Feb 17, 2019 1:31 pm

OK tube experts from a SS guy. Question: What causes the KT120 tubes to appear to "fog up" on 1 side? Is it the high voltage going to the tube nearing it's design limit? Bias set to 100. Every picture I have seen where these have been used for a number of hours this seems to appear, including mine. Inquiring minds want to know your thoughts?
CF275.jpg
CF275.jpg (47.78 KiB) Viewed 397 times
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Ben D. Rinkin
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Re: CF 275 KT 120 tubes in a fog

Post by Ben D. Rinkin » Sun Feb 17, 2019 2:15 pm

Mr. Don,

That simply is the flashing. Flashing is done in a few different ways in tubes as far as location and looks. Others will be able to word this better than me.

My understanding is the flashing is a material applied in the tube before sealing. After sealing, for a short time, a very high voltage is applied to "flash" and burn out any impurities. Flashing then left behind is able to negate impurities that burn out of the tube grids over its life. An indicator of tube life nearing it's end is little to no getter material remaining.

I hope others with greater knowledge will be along to add or correct my understanding. Some folks here seem to really know tubes.

BDR
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Don
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Re: CF 275 KT 120 tubes in a fog

Post by Don » Sun Feb 17, 2019 3:40 pm

Thanks Ben for the detailed explanation. Only have one other tube amp, the little CF 20 watt. Never have seen this on that one which is why I asked.
To argue with a person who has renounced the use of reason is like administering medicine to the dead.
— Thomas Paine


"Try not to become a man of success, but rather a man of value" Albert Einstein

"Politicians are the only people in the world who create problems and then campaign against them". Charlie Reese

"The problems we face today exist because the people who work for a living are outnumbered by those who vote for a living." Brad Shurett


"Kindness is a language that the Deaf can hear and the Blind can see." Mark Twain

"Your integrity will always be remembered longer than your prosperity."


RIP BILL D. RIP WRABBIT

UncleMeat
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Re: CF 275 KT 120 tubes in a fog

Post by UncleMeat » Sun Feb 17, 2019 3:41 pm

Yes, there are components inside the tube called "getters". The getters are coated with a metal powder before vacuum sealed, then they are heated with microwave energy to cause the powder to sputter coat the glass. The metallic spots on the glass are to attract impurities and capture them. Some impurities/remnant gas are captured when the tube is first manufactured. As the filament/cathode wears while the tube is in use the getter flash absorbs the impurity as it is released. A tube with a weak getter flash or white/chalky appearance indicates the tube is either getting near it's end of life or has failed due to a crack in the glass allowing air to get in.

The KT120 has two flashes on the sides and one on the top, the tubes in the photo are normal in appearance. If you look at the tubes in a dark room while it's playing and the black structure (the tube anode/plate) glows red, it's not good for the tube, and an indication that it's not operating within it's design limits, or the tube is getting old/tired. I've had some mild red-plating of the Svetlana 6550's at high volumes in the CF 275, so you need to be careful with over-powering the KT88/6550 type tubes.

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Don
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Re: CF 275 KT 120 tubes in a fog

Post by Don » Sun Feb 17, 2019 5:31 pm

Thanks Uncle Meat. The tube knowledge on this forum is impressive to say the least. Had no clue.
To argue with a person who has renounced the use of reason is like administering medicine to the dead.
— Thomas Paine


"Try not to become a man of success, but rather a man of value" Albert Einstein

"Politicians are the only people in the world who create problems and then campaign against them". Charlie Reese

"The problems we face today exist because the people who work for a living are outnumbered by those who vote for a living." Brad Shurett


"Kindness is a language that the Deaf can hear and the Blind can see." Mark Twain

"Your integrity will always be remembered longer than your prosperity."


RIP BILL D. RIP WRABBIT

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Scarabeo
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Re: CF 275 KT 120 tubes in a fog

Post by Scarabeo » Mon Mar 04, 2019 11:46 pm

Humm I always thought those were the marks of soles the tubes captured from all the music that played through them???

:lol:

Often an output tube has a spec of dust-like impurity, which comes into contact with an internal element, shorts the element and gets itself vaporized into gaseouse oblivion. The Getter, the mirror-like shiny plating on the inside of the tube does its job, absorbs the vaporized material and the tube is like new again.
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Ben D. Rinkin
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Re: CF 275 KT 120 tubes in a fog

Post by Ben D. Rinkin » Wed Mar 06, 2019 6:00 am

That's a great way to look at it. The soul of the tube. Cool.

BDR
Socialism takes and Capitalism creates

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