Yvon,
I find more typically, they simple don't start, but the common cause
of failure is the gradual diffusion of helium out of the tube, until there
is not enough to maintain stimulated emission. It sounds like your tube just
passed through this trainsion. In some of the early tubes the electrodes
were made from a metal that helium would diffuse through fairly easily and
these tubes had a short lifetime because of this. simply changing the
electrode metal greatly increased their life time - 26 years is impressive.
Gary
Gary Karshner
St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU