The EF86/Z729 is the standard early stage audio pre-amplifier valve that replaced the EF37A in the early 1950's.
The Z729 is as a low noise pentode intended for use as a RC coupled AF voltage amplifier, particularly in the early stages of high gain audio amplifiers, microphone pre-amplifiers and magnetic tape recorders.
The assembly is rigid to eliminate microphony and the heater is a twisted pair to reduce any external magnetic field and thus hum pick-up.
The Osram consumer brand label of this M-OV product.
The screen hides the construction but the anode can be seen as two small areas joined by a strap. Near the base the open helix of the suppressor grid can be seen. EF86 has an opening in the screen that allows a view of the grids. The upper mica is duplicated and thus has resistance to the electrodes moving. This helps to eliminate the microphonic effects found in earlier valves.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 47 mm tall.
Reference: Data-sheet & 1040. Type Z729 was first introduced in 1959. See also1959 adverts.
