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Why are 6V6G(T)'s always black inside?

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#2
Hi,

Every one I have ever seen is always black on the inside surface of the glass envelope. Even ones with relatively little use. Whyzat?
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Most older 6L6G had this coating as well.
Not all 6V6s do have that coating (graphite) though, the USSR ones for instance don't have this.

6L6 STORY

At the bottom of page are some 6V6 pictures.

Note also that not the entire glass is coated, it just grows darker with ageing because of the gettering.

Now as to why this was done, I don't know but I could try to find in explanation in my tube history books....

Cheers,😉
 
#3
I've got some new looking Magnavox 6V6s that don't have the coating. Small cylindrical ribbed plate, gray color. They look real neet.

I seem to remember the explanation is the glass they used to use was too good an insulator, so electrons which got away from the plate and struck the envelope (giving the blue glow) built up a charge, and over time this would have bad effects on the glass. So they put conductive graphite inside to provide leakage current.

Or maybe they did it to increase IR absorbtion, allowing more plate dissipation. Who knows.

Tim
 
#4
Hi,

I seem to remember the explanation is the glass they used to use was too good an insulator, so electrons which got away from the plate and struck the envelope (giving the blue glow) built up a charge, and over time this would have bad effects on the glass. So they put conductive graphite inside to provide leakage current.
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You're going to have a hard time explaining, mister.

You don't make any sense here, junior.

Cheers,😉
 
#8
Hi,

I dunno why it's dark either. Perhaps the designer had a fondness for Guinness.
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With my fondness for the occasionnal Guinness?

Rest assured, it IS the glass that made them use the coating.

Secondary emission comes to mind with impurities in the bottle...too young to quote this from the top of my head though...

Pretty sure I'm not too far off base.

Cheers,😉
 
#10
I have a whole lot of tubes that are black inside, certainly a lot of JAN ones seem to have the black coating. Maybe the army liked the blackout look 🙂

I think it was to make them look more like the metal tubes of old...
 
#11
From the book “Grondslagen van de radiobuizentechniek”, Ir. J. Deketh, Philips 1946 (Principles of Radio Tube Technology):

§4 The internal blackening of the balloon.

If electrons move outside the electrode system they can, if they hit the glass balloon or isolated parts, release secondary electrons. These secondary electrons result in unwanted disturbances, like distortion and amplification variances as a result of resulting positive remaining charge (ions) at the glass envelope and other isolated parts. Therefore often a carbon layer is applied to the surface of the glass envelope, which greatly reduces the secondary emission factor.

Cheers
 
#13
Hi,

my GE 6l6 doesn't have tt coating...and I kinda like the blue glow it gives off...looks cool...
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Thank you for the information. By posting it you just woke up 2/3 of everyone subscribed to this thread....

Would you mind refraining a little or just post in some more informative way to all involved?

This is not a hot headed member trying to chase you away, just one trying to tell you that other members aren't all that happy...

It often seems as if, when everyone else has come to a conclusion, you need to say; yes, me too.

Pretty soon we're going to have people being proud of having blue glows in their tubes or collecting tubes with carbon coatings on their bulbs...

Neither of which merits a frenzy...

Thank you,😉
 
#15
True, in comparison my Svet 6550B's with a pot getter look like clean jam jars, there isn't a dash anywhere. The darkening used to be done to KT66's. .......The best tubes for that blue glow are 807's.

The JAN designated tubes (Joint Army & Navy) were meant to be "bomb" proofed design, with better internal reinforcement and stronger electrical connections. I doubt it. Electrically they are ident to civi counterparts. I have a few JAN 6550 and they behave no different to bog standard versions.
Loook what you pay for JAN types.......a merchants rip off.
rich
 
#16
Hi,

I have a few JAN 6550 and they behave no different to bog standard versions.
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That's not quite true. Almost on all occasions these are ruggedised versions of the more common valve type sporting the W designator.

Whether this type of construction is important to your audio system I'll leave up to you.

More importantly, if you buy into NOS tubes, is to know the manufacturer and their reputation on that tube type IMHO.

Another topic, the blue glow: this is now somehow advertised as an O.K. thing to have.
At the same time it is claimed this is caused by high vacuum.

Well, too high a vacuum_and a blue glow is an indicator of that_ will reduce tubelife considerably.

Maybe something a manufacturer wants...Not something I want though.

Keep in mind also that a blue glow can be an indicator of tube going to fail any time soon too.

Cheers,😉
 
#17
I thought they exhibit lower microphonics?? aAnyways I dun like to pay for some funny stuff that I wun admire...they seem pretty standard...just relabelled JAN...I do have a few USAF 6888s that have a metal base...they do look quite strong though...with the metal base...anyone wants them?? I dunno if they can be made int oan amp or something...
 
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