From saddr–(at)–ston.ac.uk Tue May 5 08:52:59 CDT 1998
Article: 66039 of rec.audio.tubes
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From: “Russ Sadd”
Newsgroups: rec.audio.tubes
Subject: Re: Black coated tubes. Why?
Date: Mon, 4 May 1998 15:45:43 +0100
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Hiya,

We had this one a while back. You find coatings on the inside that:

- act as a getter
- give shielding
- enable better heat dissipation (black body radiation)

You’re more likely to find that the latter is the main use, external
shielding being more common. For example, the metal valves used in Britain,
America and Germany (the Germans made some superb ones in the 1930′s),
metallized coatings (such as the rich golden and red colours used by
Mullard), all helped to screen the valve. People also looked to the base and
the pin connections to give better shielding and a clearer connection (the
Telefunken doorknobs, the Mullard acorns). These casings would also enable
better heat dissipation – the Americans went for metal, the Germans ended up
with ceramic, and the British played around with silica and external
coatings.

Russ

From eyejac–(at)–acific.net.sg Tue May 5 08:53:39 CDT 1998
Article: 66160 of rec.audio.tubes
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From: Kamaruddin
Newsgroups: rec.audio.tubes
Subject: Re: Black coated tubes. Why?
Date: Tue, 05 May 1998 16:58:44 +0800
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To: Garthap
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Garthap wrote:

> >Ok you RATs. Here’s a question. what is
> >the logical reason why some tubes is
> >coated black in the inside of the glass?
> >I was given one answer which I think is
> >the most logical. I want to know your
> >logical reason. This could be a good
> >lesson to the newbies.
> >
> >One Eye Jack
> >
> >
>
> All wrong!
>
> I think it serves no other purpose than to eliminate any possible light source
> in MIL applications. I have some vintage 40′s 2A3s with this and I can’t think
> of any other possible reason.
>
> They sound exactly like “Nak Ed” 2A3s from the same manf.
>
> So? Do I win?
>
> Gartb

And the winner is…. Gartb. That’s what I’ve been told. It’s to sheild the
light in military equipment. Sounds very logical to me. No light, no target to
shoot. Shitty Chinese tubes should not have any coating on them!.

One Eye jack.

From vactube–(at)–internet.net Tue May 5 08:53:59 CDT 1998
Article: 66123 of rec.audio.tubes
Message-ID: <354E9754.398--(at)--internet.net>
Date: Tue, 05 May 1998 00:36:49 -0400
From: Jim Cross
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Subject: Re: Black coated tubes. Why?
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Kamaruddin wrote:
>
> Ok you RATs. Here’s a question. what is
> the logical reason why some tubes is
> coated black in the inside of the glass?
> I was given one answer which I think is
> the most logical. I want to know your
> logical reason. This could be a good
> lesson to the newbies.
>
> One Eye Jack

It seems that I answered this question a week or two ago. I’m a bit
appalled by everyones wacky answers, but the gray/black coating on the
inside of tube bulbs is carbon and is meant to act as an electrostatic
shield and to prevent emission from the glass induced by the electron
streams. Again I will cite my source, the 4th edition of the Radiotron
Designer’s Handbook, page 5:

“(iv) Bulbs – The inside surfaces of glass bulbs are frequently
blackened. This has the effects of making them more or less conductive,
thereby reducing the tendency to develop static charges, and reducing
the tendency towards secondary emission from the bulb.”

–
Jim Cross – Vacuum Tubes, Inc.

http://www.cinternet.net/~vactubes

email to vactube–(at)–internet.net
Check us out for new, used, audio, antique, and collectable types.

Vacuum Tubes, Inc.
3246 Floridale Lane
Cincinnati, OH 45239-6203