12-29-2013, 05:04 PM
Is the audio only distorted at low volume?
The early 90 used a 24A plate detector which gives a distorted output at low volume settings. There is no AVC, so volume is controlled by setting the bias, and therefore the gain of the RF and IF amp tubes. So when the volume is set low there is a very low level IF signal fed into the control grid of the plate detector tube. The tube is then operating in the nonlinear portion of the tube transfer characteristic near cutoff and therefore the audio output is not a faithful reproduction of the IF signal input. As the volume setting is increased, the IF signal into the detector grid becomes larger and the tube operates on the more linear portion of the grid-plate transfer curve.
Low level distortion is typical of any radio with a plate detector and a manual gain control.
The early 90 used a 24A plate detector which gives a distorted output at low volume settings. There is no AVC, so volume is controlled by setting the bias, and therefore the gain of the RF and IF amp tubes. So when the volume is set low there is a very low level IF signal fed into the control grid of the plate detector tube. The tube is then operating in the nonlinear portion of the tube transfer characteristic near cutoff and therefore the audio output is not a faithful reproduction of the IF signal input. As the volume setting is increased, the IF signal into the detector grid becomes larger and the tube operates on the more linear portion of the grid-plate transfer curve.
Low level distortion is typical of any radio with a plate detector and a manual gain control.
