[Amps] tube flatulence - gettering

[jsb at digistar.com]

Question:

I was given a homebrew single 3-400Z amplifier which works but I noticed
with ZSAC the top of the glass envelope has a very faint blue hue when
looking across the top of the tube envelope.  When looking down at the top
of the tube I don't see this color, only when looking across the top -
keying CW does not pulsate the color, the blue hue is the same regardless
of output power.  I have a couple other old junk clunker 3-400Zs that do
the same.

Is this normal or is it an indication of gas?

I have some 8163 Amperex 3-400Z in a Drake L4 that do not exhibit this
coloration, ever.  The Eimac 3-400Z tubes do, even after having been used
to produce enough output RF to turn the anode reddish-orange for 10-15
seconds.  Or does the getter not getter that quickly...



thanks

73 Jason N1SU