Once someone gives me the answer I'll feel foolish for asking-
-but-
I have been working on and with and enjoying tube equipment since the late 70s when all of the old timers thought it was mighty curious for a kid to be interested in obsolete equipment -
and, during all of that time, I have never known rhyme or reason why a relatively small subset of low/medium power output tubes - usually 6V6 and sometimes close kin like 6K6 - very commonly have an opaque black or dark gray lining on the glass. It doesn't seem common on lower dissipation tubes or higher dissipation tubes and I am at a loss why it is so extremely common on these low/medium output tubes.
-but-
I have been working on and with and enjoying tube equipment since the late 70s when all of the old timers thought it was mighty curious for a kid to be interested in obsolete equipment -
and, during all of that time, I have never known rhyme or reason why a relatively small subset of low/medium power output tubes - usually 6V6 and sometimes close kin like 6K6 - very commonly have an opaque black or dark gray lining on the glass. It doesn't seem common on lower dissipation tubes or higher dissipation tubes and I am at a loss why it is so extremely common on these low/medium output tubes.
You may be just young enough to not remember the spray of light on the wall behind a tube radio in a dark room.
Many of the things we like about tubes were seen as "faults" at the time. Metal tubes don't break and don't glow and were seen as much-improved.
Many of the things we like about tubes were seen as "faults" at the time. Metal tubes don't break and don't glow and were seen as much-improved.
I think you might be talking about the internal gray or black coatings found on some vintage octal signal (6SN7/6SL7/6C5/6J5 etc) and power tubes like 6V6G, 6V6GT, and 6L6G - typically late 1940s tubes from GE/Sylvania/Kenrad, etc.. Brimar made 6SN7 and 6J5 (considered highly desirable) with coated glass well into the 1960s. Kenrad made 2A3 (very good ones IMLE) with coated glass in the 1940s. I am not referring to getter material.
The only reason I can think of is that it prevents fluorescing of the glass in operation, possibly by absorbing some secondary electronic emission from the plate. (It's probably very moderately conductive)
The only reason I can think of is that it prevents fluorescing of the glass in operation, possibly by absorbing some secondary electronic emission from the plate. (It's probably very moderately conductive)
If that is a real electrical advantage many many more tubes would be black coated inside .
Probably the military explanation is the right answer .
Probably the military explanation is the right answer .
There's no secret to this, just pick up a book. It's literally on page 5 of RDH4. Another random title from my shelf is Deketh, 1949 Fundamentals of Radio Valve Technique. NV Philips Gloeilampenfabrieken. p48
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It costs money and therefore most manufacturers stopped using it regardless of the (small) benefit. Long ago I measured distortion in some samples of both coated and uncoated 6J5 and was not able to establish any significant difference in audio amplification which is what we are talking about here. In some critical scientific applications it might matter.
Military electronics in the U.S. at least was mostly fully enclosed so I would discount that last.
Thanks for all of the interesting responses and information. I'm finding PRR's especially "illuminating" - because the era of octal-base-tube consumer sets using tubes in the 6v6 class was probably the height (and the end) of fully-open-backed radios and TVs (that would tend to throw a lot of light out of the open back), and the 6v6 was also in the era in which there were metal vs glass options. It still seems curious that the 6V6s produced for years later often continued to have the dark coating even as many other tube types lost (or never had) the coating. I guess the practice just continued.
And agree with KevinKR, any milspec vacuum tube electronics (another interest of mine) had the tubes deep behind opaque (and for field equipment, also airtight and watertight) enclosures.
Again, thanks for the interesting info and observations.
And agree with KevinKR, any milspec vacuum tube electronics (another interest of mine) had the tubes deep behind opaque (and for field equipment, also airtight and watertight) enclosures.
Again, thanks for the interesting info and observations.
From Deketh , Fundamentals of Radio-Valve Technique, Page 48.
"29. Internal Carbonising of the Bulb
When electrons pass outside the electrode system they may strike
against the glass walls of the bulb or against insulated parts of the
valve, and release secondary electrons therefrom. This secondary
emission produces various undesirable effects such as signal distortion
and fluctuations in the degree of amplification, which can be ascribed
to positive charges in the glass wall, as explained in Chapter XXXI. In
order to minimize these effects, the interior surface of the bulb is
usually coated with carbon, which, as stated in a previous chapter, has
a low secondary-emission factor."
"29. Internal Carbonising of the Bulb
When electrons pass outside the electrode system they may strike
against the glass walls of the bulb or against insulated parts of the
valve, and release secondary electrons therefrom. This secondary
emission produces various undesirable effects such as signal distortion
and fluctuations in the degree of amplification, which can be ascribed
to positive charges in the glass wall, as explained in Chapter XXXI. In
order to minimize these effects, the interior surface of the bulb is
usually coated with carbon, which, as stated in a previous chapter, has
a low secondary-emission factor."
Oops , just saw I missed Merlin's post.
Also wonder if use inside mil. radio tubes helped with heat. On metal anodes, blacking reduces temp by sixty to eighty percent.
Also wonder if use inside mil. radio tubes helped with heat. On metal anodes, blacking reduces temp by sixty to eighty percent.
It's interesting, though, about the idea of the glass and secondary emission, that any number of later and much more powerful tubes have no coating on the glass
They developed conductive glass that didn't need the carbon anymore, and improved electrode geometries with not so many gaps where the electrons could escape.