The 12BA6 is a variable μ pentode for use at HF within AC/DC receivers. Most probably as RF amplifiers. The 300 mA heater is clearly rated for series heater chain use.
The brand name RCA Victor and made in Chile.
The RCA Logo.
The bottom mica components. The vertical screen on the left shields the anode connection.
The thin glass tube envelope is 18 mm in diameter and excluding the B7G base pins, is 47 mm long.
References: Data-sheet & 1040. Type 12BA6 was first introduced in 1946. See also1946 adverts.
