British versions of the adapter use 6F33 mixer pentodes for V3 and V4. North American versions (RA37B)
use 6AS6 mixer pentodes and omit the grid-stopper resistors R35, R36, R37, R38. Mixer pentodes, also
called 'dual control' or 'short suppressor base' pentodes, differ from normal pentodes in that grid 3 has a
close wire spacing and is internally connected to a diode (between grid 3 and the cathode) which
prevents grid-current. The 6F33 and 6AS6 are not pin compatible (g2 and g3 connections are swapped)
and different cathode resistor values are used (see electrical differences table above).
The balanced mixer has a broadband output. The anode load (L19//R27) is broadly tuned to 2.7MHz,
which is the logarithmic centre of the 2 to 3 MHz output range. There is no resonating capacitor because
the coil is tuned by self and stray capacitances. To measure the frequency respons of the stage, inject a
signal into V3 grid 1 ((pin 1) with the antenna disconnected, and measure the output across a 330Ω
terminating resistor connected across the RF output socket. Adjust L19 for maximum output at 2.7MHz.
Mixer balance adjustment is by RV1 (front panel preset) and C28, set by tuning the RA17 interpolation
receiver to 2MHz (0 on the red scale) and minimising the carrier level.