The sidetone output of the T-368 is taken through a cap from the wiper of the 500 Ohm cathode resistor of the cathode follower. Is it normally connected across the "output" side of the 600 Ohm audio output of the R-390? If so then the level of the sidetone is controlled by the cathode resistor independ from the R-390? K3PID - Ron H.