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Posted: Wed Sep 29, 2010 12:14 am
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Grey glass on the inside of the tube is graphite. It supposedly helps absorb stray electrons. Also, helps absorb heat. Common in 1940-50's tubes.

Tonewise, I find that gray glass 6V6GT have a "browner" tone. Slower onset of overdrive. I like gray glass tubes better. Even in gain stage tubes, like 6SN7GT's.

This is not to be confused with Russian-made tubes with solid black color inner glass surfaces. This is not graphite, but paint. I was told that many mil spec Russian tubes had black paint on inner surface to help avoid outside electro-magnetic interference, say from a US device shooting laser beams at missiles with these tubes to disrupt their function.


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Posted: Wed Sep 29, 2010 5:09 am
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I've said this before but my first amp was a tweed Champ and I really liked it,but it developed a problem which most likely would have been a simple fix,but there was no amp tech in the small town I lived in,only the tv repair (hack) man....he opened it up and tried something that didn't fix it,and then later on my uncle who had given it to me said he would try to find somebody to fix it in or around Atlanta,that's where he lived at the time.
It left my posession and he lost track of it....at the time,35/40 years ago,it didn't seem like a big deal,I mean it was ONLY an old Fender tweed amp. :roll: ...I was into bigger and better things,now it makes me sick,I should have just hung onto it and then in later years I gained the knowledge on how to fix some things,or find somebody who knew.
Then the whole vintage thing happened and the internet was invented and here we are still talking about '50s Fender technology.
Man,who knew. :wink:


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Posted: Wed Sep 29, 2010 10:18 pm
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BMW, that is some great info ! Always thought the color was for heat dissipation. Didn't know about the graphite coating being used for electron absorbtion. I wonder if that was a hold over from the metal tubes ? The black painted Russian tubes is something else I was never aware of. Art

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Posted: Thu Sep 30, 2010 12:49 am
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Art, I was told that the graphite coating was a byproduct of WW2 radar and radio technology. To try and reduce stray electrons and electro-magnetic coupling and other interference between components.

A nice side-effect is a tube which has better midrange and slower onset of overdrive (in general). When compared to clear glass tubes, the clear glass will usually have cleaner highs and perhaps tad better bass. At the cost of the silky midrange. And breakup a bit harsher than the smoked (or graphite coated) tubes.


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