I recently restored a 1933 Emerson 250AW small superheterodyne set which uses a 77-tube as a plate detector. My question is: Is there any sort of trial & error method of varying the voltages or components to reduced the audio distortion? It's not terrible and I realize this is a non-linear detector. The sound improved somewhat when I replaced the 15K cathode bias resistor with a 10K unit. Adding a small rf-choke to the plate lead did nothing as did adding a 0.5mfd bypass cap to the screen. Just wondering if there was someway I could "tweak" this detector to minimize distortion.

Hi Rich

Check the screen bypass cap for leakage along with voltages on the 77 tube. The plate, pin #2, should be 35 volts. Screen, pin #3, 25 volts and cathode, pin #5, 1.5 volts. You shouldn't need to modify the circuit but proper voltages are necessary for distortion free sound.

Norm

: I recently restored a 1933 Emerson 250AW small superheterodyne set which uses a 77-tube as a plate detector. My question is: Is there any sort of trial & error method of varying the voltages or components to reduced the audio distortion? It's not terrible and I realize this is a non-linear detector. The sound improved somewhat when I replaced the 15K cathode bias resistor with a 10K unit. Adding a small rf-choke to the plate lead did nothing as did adding a 0.5mfd bypass cap to the screen. Just wondering if there was someway I could "tweak" this detector to minimize distortion.

Hi Rich, The biassed detectors give minimum distortion when the cathode is bypassed with a small value of capacitance, about 300 to 500pf for the value of cathode resistor in your set. A large value of bypass capacitor is often used to increase the gain at the expense of distortion. If you've got plenty of gain you could try reducing the capacitor and see how it works. Note that the set is no longer original but should sound better if there is sufficient volume. This is covered in the Radiotron Designers Handbook, pages 1084/85 in my fourth edition. Don Black.

: I recently restored a 1933 Emerson 250AW small superheterodyne set which uses a 77-tube as a plate detector. My question is: Is there any sort of trial & error method of varying the voltages or components to reduced the audio distortion? It's not terrible and I realize this is a non-linear detector. The sound improved somewhat when I replaced the 15K cathode bias resistor with a 10K unit. Adding a small rf-choke to the plate lead did nothing as did adding a 0.5mfd bypass cap to the screen. Just wondering if there was someway I could "tweak" this detector to minimize distortion.