>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?
** Correct. Reducing filament-V automatically c. 3% during Rx makes
economic sense. The amplifier shown in:
http://www.somis.org/PBox.top.inside.jpeg
has this feature.
> Ouch. How long does the 3-500Z
>tungsten-dicarbide surface layer last?
** Depends on filament temperature. Each 3% increase in filament V
above what is needed to obtain normal peak emission results in a 50%
reduction in emissive life. In algebraic terms, emissive life is
proportional to (E1/E2)^23.4. The bottom-line is that by maintaining the
minimal filament V necessary to obtain normal emission, a life of c.
22k-hours is do-able.
>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.
** Cycling filaments is no problem if one uses a stepstart such as:
http://www.somis.org/D.a.01.GIF
- Another solution is to use a current-limiting filament tranformer like
is utilized in the Heath SB-220.
>
>> >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.
** Indeed they are. As a result, when and if VHF regeneration takes
place, it runs amuck inside the amplifier.
cheers, Jason.
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