14-11-2015, 12:33 PM
What is often overlooked when considering the leaky grid detector is that the valve is actually performing as two different valves.
Although the entire valve, be it a triode or pentode, is acting as an amplifier as might be expected, the grid/cathode (or filament) section of the valve is acting as a diode at the same time!
The incoming modulated RF signal swings equally positively and negatively about the zero point which cancel out and, therefore, leaves an average level of zero. To obtain any information from the signal we need to get rid of either the positive or negative portion of the signal.
In the leaky grid detector, because there is no bias, as soon as the grid sees a positive going signal it acts as the anode of a diode and conducts, leaving only the negative portion of the waveform. The average of the negative going signal is now the effective grid bias but this will automatically track the incoming signal level, being bled to earth via the grid leak.
Note that this process leaves the AF modulation intact to be amplified by the valve in the usual way and the residual RF is filtered out in the anode circuitry.
Once the audio signal has been recovered, it is important not to distort it. In later stages, therefore, grid bias is applied to prevent any part of the signal pushing the grid positive relative to the cathode (or filament)
Although the entire valve, be it a triode or pentode, is acting as an amplifier as might be expected, the grid/cathode (or filament) section of the valve is acting as a diode at the same time!
The incoming modulated RF signal swings equally positively and negatively about the zero point which cancel out and, therefore, leaves an average level of zero. To obtain any information from the signal we need to get rid of either the positive or negative portion of the signal.
In the leaky grid detector, because there is no bias, as soon as the grid sees a positive going signal it acts as the anode of a diode and conducts, leaving only the negative portion of the waveform. The average of the negative going signal is now the effective grid bias but this will automatically track the incoming signal level, being bled to earth via the grid leak.
Note that this process leaves the AF modulation intact to be amplified by the valve in the usual way and the residual RF is filtered out in the anode circuitry.
Once the audio signal has been recovered, it is important not to distort it. In later stages, therefore, grid bias is applied to prevent any part of the signal pushing the grid positive relative to the cathode (or filament)
