The AC/VMSG12 is a mystery. The only information found so far is that contained on the bulb. So it appears to be a variable μ screen grid valve or tetrode with a 4.0 Volt heater. The 4.0 Volt suggest indirectly heated. The screw top cap connection suggests early to mid 1930s.
The valve appears to be designed for use in a superhet receiver in the UK where the 4.0 Volt heater was standard.
The screw top cap connection.
The classic envelope is 49 mm in diameter, and excluding the B5 base pins is 133 mm tall.
Absolute Maximum Operating Conditions¶
Updated September 10, 2013.