The EF91 was almost the universal valve of the early 1950s, as can be seen by the number of manufacturers who made versions of this standard valve. The first reference we have to the EF91 is the review of the 1947 Olympia Show in Wireless World. This exhibit is from the 6202 Valve Company.
This makers's implementation does not have the coating inside the envelope to reduce the gas evolved by electron bombardment.
The anode consists of two small plates joined by a strap - here seen on the right. The suppressor grid extends beyond the anode. Screening can be seen below the lower mica and above the top mica.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
References: Data-sheet & 1040 Type EF91 was first introduced in 1947. See also1947 adverts.
