Richard is quite correct, tho to older hams, believe the
term he uses for "Scavengers", were called "GETTERS", and
were to claim any free gas's in a tube (this is anchient
history!). And, another method that was used , was an
"Extender", which was in reality, nothing more than a step-
up transformer for the filament, raiseing the voltage (and
hence the cathode temperature) by 1 to 2 tenths of a volt.
these were also (a variation of this principle) available
on telephone microwave carriers , where the klystrons cost
several hundreds of $$$ . These, normally set the voltage
at 6.3 volts, but you could (at your own risk) turn it as
high as 7 volts. But, normally didn't exceed 6.5 volts.
and to give an idea just how prudent, the receive local
oscillators is some of that equipment, (according to the
date codes), lasted in excess of 20 YEARS continious service!
It wasn't the phospors that deteriorated, but the free
electrons that the cathode could radiate!
And, as a side note, Southern Pacific, found an outfit that
would replace the filiment/cathode and rebuild those same
klystrons , at about 1/4 the price of the new ones- the
original equipment was made by Lenkurt , about 1960 vintage
and only , about 8 years ago was finally retired, replaced
with solid state, mostly digital, but some analog equipment!
If this hasn't boored you by now, dont know what it will take!

As info, Jim NN7K (really dates me) !

Richard Clark wrote:
On Fri, 29 Apr 2005 09:05:27 -0700, "John Smith"
wrote:


A crt fails/dims due
to air entering the tube, possibly?


Hi Brett,


Principally it is the oxidation of the cathode (one reason why tubes
have scavengers) from what air had remained from the production cycle.
Snip
Back when I did TV repair as a teen, and then later in the Navy as
part of a Christmas charity project, we used what was called a
"rejuvenator."
snip
This piece of gear would apply several hundred to a thousand volts
between the cathode and the first grid (usually a cylindrical shield
capped with a plate with a pinhole in it) and hold the voltage there
for a couple of seconds (manufacturer's recommended time). This would
produce a current that would rip the surface of the cathode open
revealing a new emitting area and the tube would come to life for
another 6 months.
snip again!
73's
Richard Clark, KB7QHC