"TerryJ" ) writes:
I am considering adding a product detector to my S40A. Has anyone used a
6SN7 to replace the 6J5 BFO and feed the IF signal to the grid of the second
triode with the cathodes tied together? The original oscillator circuitry
would be wired to the first triode. Audio from the plate resistor of the
second triode (after IF filtering).

You'd be wanting QST for April 1954. "Modifying the S40 for S.S.B. Reception"
by Edward Sommerfield. It's also reprinted in the ARRL's "Single Sideband
for the Radio Amateur", first edition which came out in '54.

He adds a crystal filter for selectivity. Then a third IF stage to compensate
for the loss in the filter. He injects the BFO into the grid of this stage,
complete with a front-panel control for injection level. He changes the
detector to a 1N34 diode, but it's still an envelope detector.

He even shows a means of modifying the existing BFO for crystal control.

At a glance, he also ads a stage of audio, to componsate for the (likely)
use of the RF gain control to cut back on gain.

He adds a VR tube to regulate the voltage going to the local oscillator
and BFO. He even adds a tuning eye tube.

Michael VE2BVW