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The SG Valve
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This is a mystery valve. This Cleartron does not match the Cleartron company we know of either in Type designation or logo. The UK subsidiary of the American Cleartron company operated between 1925 and 1927. We may assume that Cleartron is not the company name but a brand.
The 215SG is a 1929 screen grid valve with a box like screen grid and a top cap. This has no top cap and the grids are not of 1920s screen grid style.
The side contact to make the fifth contact on this battery directly heated valve. The B5 base was not introduced until 1928.
The paper band across the top of the valve has been damaged but seems to give the anode voltage as between 90-150 Volts and the auxillary grid at 60-100 Volts. This would be in keeping with a screen grid valve or tetrode.
The anode is is flattened cylinder and has been left bright as this is a low power valve.
With the aid of the microscope, the grids were seen. The second grid is quite closely wound compared to the inner grid and for a tetrode a considerable advance on the design used in the late 1920s and early 1930s.
Here the filament can be seen as well as the grid geometry
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 84 mm tall.
Reference: Observation.


Updated August 17, 2017.
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