![](../static/b5-dim.jpg)
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.
![](../static/b5-1.jpg)
![](../static/b5-2.jpg)
An unusual square surface mounting B5 valve holder.
The pin-outs below refer to the data researched by Keith R Thrower OBE.
![](../static/baseb5t.jpg)
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