This EF85 exhibit happens to have the yellow paint that Mullard used to celebrate thair Goldern anniversary in 1969 - gold paint being deemed too expensive.The EF85 is a VHF varible μ pentode and is the counterpart to the EF80 straight RF pentode. Clearly this places the valve in a superhet context with AGC applied to the RF and IF sections. At the time it was designed television receivers operated on Band I at about 50 MHz and from 1955 also on Band III at around 175 MHz. The EF183, released in the 1960s, overtook its performance just as the EF184 replaced the EF80.
The EF85 was tasked with IF amplification in VHF/AM radio receivers.
When Band IV and Band V UHF television entered service in the early to mid 1960's the front end required specialised triodes to operate at 400 - 850 MHz.
Like the EF91 and EF80 the anode is in two plate sections joined by a thin strap. It is a true pentode and the three grids can be seen inside the perforated screen.
The screen is extended below the lower mica and shields the anode pin connection.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 56 mm tall.
References: Data-sheet & 1040. Type EF85 was first introduced in 1951. See also1951 adverts.
