The TH2321 is a triode heptode frequency changer for 200 mA series heater chains. The TH233 is the same basic design but with lower internal capacitancies and the newer MO base.
The reverse is plain zinc paint with the two code letters EW.
The classic envelope is 38 mm in diameter, and excluding the B7 base pins is 103 mm tall.
References: Data-sheet, 1040 & 1043. Type TH2321 was first introduced in 1938. See also 1938 adverts.
Absolute Maximum Operating Conditions¶
Updated November 27, 2014.