Question regarding the "Blue Glow" of Power Tubes

[C***@hotmail.com]

I noticed last night that the 'blue glow' in my KT66 tubes intensifies
when
playing at louder volumes. The is I can control the light show by my
playing dynamics.

I never paid much attention to this with other amps I bought. But since
I built this amp
from a kit I'm wondering if this is normal.

KT66's are biased at 19 watts (Vp = 450 Vdc, Ip =42 mA).

[Tony Hwang]

Post by C***@hotmail.com
I noticed last night that the 'blue glow' in my KT66 tubes intensifies
when
playing at louder volumes. The is I can control the light show by my
playing dynamics.
I never paid much attention to this with other amps I bought. But since
I built this amp
from a kit I'm wondering if this is normal.
KT66's are biased at 19 watts (Vp = 450 Vdc, Ip =42 mA).

Hmmm,
Gassy tube? Ionization.

[The Librarian]

Blue Glow -- what causes it?

Glass tubes have visible glow inside them. Most audio types use
oxide-coated cathodes, which glow a cheery warm orange color. And
thoriated-filament tubes, such as the SV811 and SV572 triodes, show
both a white-hot glow from their filaments and (in some amplifiers) a
slight orange glow from their plates. All of these are normal effects.
Some newcomers to the tube-audio world have also noticed that some of
their tubes emit a bluish-colored glow. There are TWO causes for this
glow in audio power tubes; one of them is normal and harmless, the
other occurs only in a bad audio tube.

1) Most Svetlana glass power tubes show FLUORESCENCE GLOW. This is a
very deep blue color. It can appear wherever the electrons from the
cathode can strike a solid object. It is caused by minor impurities,
such as cobalt, in the object. The fast-moving electrons strike the
impurity molecules, excite them, and produce photons of light of a
characteristic color. This is usually observed on the interior of the
plate, on the surface of the mica spacers, or on the inside of the
glass envelope. THIS GLOW IS HARMLESS. It is normal and does not
indicate a tube failure. Enjoy it. Many people feel it improves the
appearance of the tube while in operation.

2) Occasionally a tube will develop a small leak. When air gets into
the tube, AND when the high plate voltage is applied, the air molecules
can ionize. The glow of ionized air is quite different from the
fluorescence glow above--ionized air is a strong purple color, almost
pink. This color usually appears INSIDE the plate of the tube (though
not always). It does not cling to surfaces, like fluorescence, but
appears in the spaces BETWEEN elements. A tube showing this glow should
be replaced right away, since the gas can cause the plate current to
run away and (possibly) damage the amplifier.
PLEASE NOTE: some older hi-fi and guitar amplifiers, and a very few
modern amplifiers, use special tubes that DEPEND on ionized gas for
their normal operation.

-Some amps use mercury vapor rectifiers, such as types 83, 816, 866 or
872. These tubes glow a strong blue-purple color in normal use. They
turn AC power into DC to run the other tubes.

-And occasionally, vintage and modern amplifiers use gas-discharge
regulator tubes, such as types 0A2, 0B2, 0C2, 0A3, 0B3, 0C3 or 0D3.
These tubes rely on ionized gas to control a voltage tightly, and
normally glow either blue-purple or pink when in normal operation. If
you are unsure if these special tubes are used in your amplifier,
consult with an experienced technican before replacing them.
http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm

[The Librarian]

Tubes operate on the principle of thermionic emmission. A heater
filament heats the cathode until it glows a dull red. The cathode is
made of materials that can emit electrons at high temperature. The
tubes plate is held at a high positive voltage, which attracts the
electrons emitted from the cathode. In amplifier tubes, the flow of
electrons is controlled by one or more fine wire screens or grids
placed in between the cathode and plate. The flow of electrons is
controlled by varying the voltage applied to the grid. A small change
in grid voltage results in a larger change in electron flow through the
tube, amplifying the signal applied to the grid.

Many materials flouresce or "glow" when struck by electrons. Not all
the electrons in a vacuum tube strike the plate. A few stray off and
strike other things in the tube, such as the glass envelope. Many types
of glass will give off a faint blue glow when bombarded by electrons of
sufficient energy. This is the most common type of blue glow seen in
power output tubes. Often you will see small patches of glow that
correspond to the holes in the plate or other structures in the tube.

Another type of glow you may see in power tubes appears on the inside
surface of the plate structure. The pattern of the glow will aften
appear in strips that vary along with the signal applied to the tube.
(i.e., the music.) This type of glow is generally normal as well. It is
often caused by a thin layer of atoms that have boiled off the cathode
and deposited on the surface of the plate. What you see is the slight
flourescence of this material when it is hit by electrons from the
cathode. The stripes correspond with the alignment of the grid wires.
The varying widths of the stripes demonstrate the operation of the
tube. The grid allows restricts the size of the electron beam according
to the grid's voltage.
http://members.aol.com/larrysb/blue_glow.html

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Of course, you might have a serious situation if the blue glow looks
like this.

Loading Image...

[Z]

Thanks...