On Tue, 9 Dec 2003, R. Measures wrote:
> >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.
Okay - I had read it the wrong way then. I read it as though the energy
striking the plate (and not the movement through it) was creating the
heat. I wasn't totally sure about that.
> >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 .
Wow - so regardless of transmitting or any other use, while the filament
is lit, the clock is ticking? Ouch. How long does the 3-500Z
tungsten-dicarbide surface layer last? 5,000 hours? That's only 208
days, not taking into account the cycling of powering on and off the
filament. I used to leave my old Drake gear on for a couple days at a
time to avoid cycling the filaments - that's disappointing if I have to
weigh the differences between cycling the filament (which to me is pretty
bad for it) or leaving them lit for 48 hours at a time, etc.
> >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"]
I thought all "proper" modern amplifier designs were geared to eliminate
VHF from ever making it beyond the amplfier chassis.
thanks,
Jason
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