>Hello,
>
>as far as I could remember, several articles cover that topic in 14th.
>edition of Hints&Kinks by ARRl.
>Basically the short, could be removed by putting appropriate amount of AC
>voltage between the grid an cathode. A bulb is in serious with ac/variac and
>then by gentle turning and light pounding with wooden stick on surface, the
>short could be removed by welding/ dewelding?/ effect. Tube should be
>wrapped in towel and of course extreme caution should be paid off.
>Please refer to the original article. Please remember : SAFETY FIRST!!
>
? A new, cold, 3-500z has a grid-filament B.V. of c. 8000v. When the
hot (1820ºK) filament is bent by a burst of grid-current during an
intermittent self-oscillation, it can touch and sometimes weld itself to
the grid. Although the weld can be vapourized by brute electrical force,
and the tube then looks unshorted on an ohm-meter, the filament is still
too close to the grid, thusly the tube may not be reliable. The best
test is to high-pot the grid-filament. Anything less than 5000B.V. is
not good news. Another test is to measure the grid-filament capacitance.
More than 20% above rated capacitance indicates a bent filament helix.
? Murphy is right: "Everything is more complicated than it first
appears".
cheers, Nermin
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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