[Amps] 3-500Z plate durability

[R.Measures]

>
>We got a good rousing set of replies on how to wire 220V, maybe we can
>strike gold again on this one...
>
>
>3-500Z plates - I'm reading that they "enjoy" getting dull red hot.
>
>My first ham gear was a Drake T4XB and R4C B-line and I distinctly
>remember being warned by my grandfather that the best way to ruin 6JB6
>tubes was to let them even slightly get red - because at that point they
>were essentially worthless for very much output.
>
>So what prevents 3-500Z or 3-500ZG tubes from failing prematurely when
>used in the manner i've been reading about, running them dull cherry red?
>
**  The materials used (C or Ta) have a higher melting point than 
orange-yellow heat.

>I read that the heat generated by the kinetic energy of electrons hitting
>the plates is what creates the heat (not the plate current), 

**  Electron movement is "plate"/anode current.

>and this heat
>"activates" the zirconium getter but what prevents the plate from dying
>like the old sweep tubes?
>
>
>Do 3-500Z tubes get "soft" or do they just suddenly drop output and fail?

**  All thoriated-tungsten tubes gradually loose emission as the carbon 
in the filament's tungsten-dicarbide surface layer is used up while the 
filament is lit  .

>And do they fail because of high grid current causing the grid to warp and
>short or because of some other reason?

**  3-500Zs can fail from a sudden burst of grid-current during an 
intermittent VHF parasitic-oscillation, but only if the Pi-network tank 
is of a low pass design so that the VHF energy being generated can not 
escape to the antenna.  In this case, the burst of grid-current from the 
unloaded condition can cause the unsupported center of the 3-500Z 
filament/cathode to be bent sideways by the perpindicular EMF and touch 
the grid-cage.  [see photo in September/October 1990 *QST*, "Parasitics 
Revisited"]
>
cheers, Jason