>
>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
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