On Mon, 4 Apr 2011 16:27:32 +0000 (UTC), Meat Plow
wrote:
On Sun, 03 Apr 2011 19:34:29 -0700, Jeff Liebermann wrote:
On Sun, 03 Apr 2011 20:09:10 -0700, David Nebenzahl
wrote:
So where does that purplish glow come from? I've seen it.
Impurities in the glass envelope or gas in the tube.
If gas, take an anti-acid tablet and you should be fine in a few
minutes.
http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm Scroll down to:
C. Blue Glow -- what causes it?
for more details.
So even after a getter flash there is still a certain amount of
impurities? I've never seen a 6L6 that didn't have some blue inside
when in operation. It looks as though the blue only occurs where the
electron beams hit the inside of the glass. At least in new tubes.
Yep. As I vaguely recall (which means I didn't Google for a
reference) glass has the irritating habit of collecting impurities and
volatiles during manufacture. Irving Langmuir figured this out in the
1920's working on light bulbs. He would draw the best vacuum possible
and in a few days, find the bulb full of water vapor and other gasses.
The hot filament would break down the water into hydrogen and oxygen.
The oxygen would then oxidize the electrodes, and blacken the inside
of the bulb.
So, he invented a method of baking the glass to remove the volatiles
prior to evacuation and an acid bath to remove some of the impurities.
It also works nicely for vacuum tubes, but like all such processes, is
far from perfect. There's always some impurities left behind.
When we talk about a "gassy" vacuum tube, it's not a leaky glass
envelope or seal. The getter does best with reactive gases and does
nothing for eliminating noble gases, which will not react with metals
(or most anything else). It's those noble gases that remain (helium,
neon, argon, krypton, xenon, and radon) that you're seeing glow. Most
likely helium and argon mix. The small helium molecule will also
diffuse through the glass from the outside air. See photos at:
http://en.wikipedia.org/wiki/Noble_gas#Discharge_color
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558