(from TM 11-866)

13.  FIRST DETECTOR. -- The 1st detector employs a VT-87
pentagrid mixer (V3).  Its injection grid (grid No. 3) is coupled to
the HF oscillator cathode, and its signal grid (grid cap) is coupled to
the plate of the second RF amplifier tube (V2) by means of the
second RF transformer.  When the receiver is tuned so that the HF
oscillator (V4) generates an RF voltage exactly 465 kc higher in
frequency than an incoming signal being amplified by the second
RF amplifier (V2), these two RF voltages are mixed together in the
first detector (V3).  This mixing process results in the generation of
a very complex waveform in its plate circuit.  One of the components
of this complex wave has a frequency of 465 kc (equal to the difference
between the two RF voltages).  This, the desired component, is
selected and amplified by the resonant step-up of the tuned circuit
C21, L26 (in T1).  The remaining components, (mostly higher in
frequency) are by-passed by C21 and C18A in series.  Any modulation
of the incoming signal carrier is faithfully reproduced in the
465 kc output of the first detector.