The CV3909 (commercially the 6BJ6) is a variable μ RF pentode designed for broadcast radio receiver applications. A key feature is the modest heater power making it attractive for mobile applications.
Little can be seen. The control grid supports are copper and the anode lead is screened.
The suppressor grid is close wound making it suitable for use as an addition control electrode.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 45 mm tall.
References: Data-sheet, 1040 & 3002. Type CV3909 was first introduced in 1947. See also1947 adverts.