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Base B5

    
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This base, a B4 with a centre pin, was introduced in 1928. Originally the centre pin was used for the cathode connection in indirectly heated valves.

In the original pin layout given to the R-type and subsequently used for countless later designs: Pin one connected to the anode, pin two was for the grid and pins three and four were for the filament.

An unusual square surface mounting B5 valve holder.
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