[Radioroslyn]

Hi TN,
The switch on the 296 is doing two things. It's changing the voltage on the 24's suppressor grids to lower the gain of the rf amp stages and switching audio input.
The thing that is a little tricky is that  #25 is part of the avc circuit so we want to leave it a lone. The original magnetic cartridge presents a low resistance across the amplifier input. Your modern cartridge doesn't. So what to do? Add a 500k -1M resistor to the control grid of 27 tube to chassis ground. Break the connection at #24 and 26. Add switch with the wiper connected to 26 (that's the audio input) One end of the switch goes to 24 (radio) and the other end to phono cartridge (phono) though a mylar cap a .005-.01 is fine.

Terry