-------------------------------------------------------------------------
A DEEPER LOOK AT THE PHENOMENON OF CATHODE STRIPPING IN THERMIONIC VALVES
-------------------------------------------------------------------------

                                                From a rec.audio.tubes
                                                article posted Dec.'96
                                                By: J.H. van de Weijer


One may assume that a thermionic valve's susceptibility to the stripping
phenomenon primarily depends on its cathode's design, and indeed that the
side effects caused by this are most prone to be seen in high grid
impedance low level signal tube circuits and their aging behavior:

Indirectly heated (generally Nickel) cathodes coated with a rare earth 
metal oxide electron emission "cement" compound are prone to mechanical 
stress caused by thermal cycling; i.e.: the heating and cooling of the 
cathode. (Manufacture dependent).

The ceramic nature of the indirectly heated cathode's emissive coating
with its (from the metal cathode carrier) differing thermal expansion
coefficient may cause surface material to crack and become "loose". The
thus gradually "powdered" ceramic cathode emission surface may keep
minute amounts of electrical charge stored after cooling down; the
surface in cold state remains nonconductive. Minute amounts of these
cathode borne particles, either with remaining charge or electrically
polarized upon sudden apply of anode voltage, may "dust off" and clog
onto the most nearby "sieve" i.e.: the control grid; cathode stripping
has happened, and here it is that this less heard of tube
degradation/aging mechanism (not discussing others) occurs!

Consider this:

The grid clogged particles due to radiant heating from the nearby cathode
will start to behave as pointwise cathodes themselves, causing beyond
normal grid current, this has the effects of:

-Drift in those high impedance biased control grid circuits: And this
 just in the unwanted direction: Take a tube endstage which is
 capacitive coupled  from the phase  inverter and DC biased through (say)
 50 kOhms: Current runs from the anode into the control grid and
 therefore shifts the grid bias voltage to a less negative value...
 There you go...
 (Ever wondered why some tube manufacturers specify a maximum grid bias
  resistance?)
-Causing excess noise.
-Etcetera, you don't want to know.

Now, how to be most gentle to your indirectly heated tubes and give them
a long life: (and this also applies for all fellow guitar players having
a "stand by" switch at hand):

SWITCH ON:
 Switch on from standby mode: i.e. only fire the filaments, wait somewhat 
 longer  than fully "glown" up, then switch from standby to power (B+).  
 (i.e.: B+ may be suddenly switched on, but only after full filament 
 warm-up)  (with regards to "cathode stripping": The cathode is now 
 conductive: All localized cathode charge will have drained).

SWITCH OFF:
 The same sequence reversed: I.e.: Turn off B+, wait, and only then turn
 off the filament supply; This will gradually and properly discharge all
 charges.
 With regards to cathode Stripping; this will assure no charge will
 remain stored locally on the susceptible cathode surface and so forth...


                                     Copyright 1996 J.H. van de Weijer

                                     This article may be freely distributed,
                                     stored, copied and printed for non-
                                     commercial use, provided the integral
                                     text including this notice is kept
                                     unmodified.
 
----------------------------------------------------------------------------