The KT61 is an audio output valve and features the standard 6.3 Volt heater that was introduced from America in the mid 1930's. The KT61 forms part of a series of very successful valves from M-OV.
The plain reverse where the Osram or Marconi labels would be stuck.
The top mica: cooling fins are attached to the control grid supports and an S shaped band connects the beam forming plates to the cathode.
The classic envelope is 44 mm in diameter and, excluding the IO base pins, is 102 mm tall.
References: Data-sheet, 1043 & 1040. Type KT61 was first introduced in 1939. See also1939 adverts.
