[R-390] IERC Tube Shields et. al.

[Drew Papanek]

Bob Camp wrote:

>Now for the complex part. Tubes wear out for a bunch of reasons:
>
>1) Filaments go open circuit
>2) Gas
>3) Filaments loose emission
>4) A grid goes open or short.

<snipped>

Don't forget other common failure modes: heater to cathode short, grid 
emission.

Grid emission results from electron-emitting coating migrating from cathode 
to grid.  In applications with appreciable grid circuit resistance this 
emission will cause grid to go positive and upset bias point.  In our 
beloved R-390 series one IF tube having grid emission can pull AGC line, 
decreasing or defeating gain control action and causing all controlled tubes 
to draw excess plate current.

Drew

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