03-02-2017, 08:03 PM
(03-02-2017, 05:42 PM)Nowhere-Man Wrote: Variable gain in the cathode circuit of an IF valve. This used variable resistance between chassis and cathode. I briefly looked up more information and someone stated it drives the cathode into more positive value and increases gain. The IF here, though, was 1700 KC.
AGC is achieved by driving the grids of the controlled stages negative, the negative voltage being derived from the detected signal.
For a manual gain control it is easier to drive the cathode positive as it doesn't need a special negative supply. As Lawrence says, as far as the valve is concerned it sees the grid as going negative with respect to the cathode. We tend to measure with respect to earth so we see the grid voltage as negative (if it is a AGC controlled stage) and the cathode as positive but the valve has no direct connection with earth (ignoring any internal shield) so the cathode is its only point of reference.
The 1700kHz IF for a communications receiver is not unusual - it improves image rejection - and 1.6MHz is also common.
What the strange book you are using? 'Back biasing' is, presumably, what anybody else would call reverse biasing.
