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Old 14th Nov 2017, 9:09 pm   #69
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
Default Re: Oh No! Not another howler.

Some valves were designed to run like that, to be honest. It sounds crummy, and it is... But it keeps things simple.

As for the signal being strong enough to overcome AGC bias, well if it's that strong, it'll generate more AGC! There is a fixed ratio between IF signal voltage, and AGC voltage, determined by the gain in the stage, the IFT ratio, and the detector. Whether this ratio is enough to always guarantee the AGC bias being sufficient to stop the grid going positive on the signal peaks, is another matter.

Adding a bit of R will give a bit of negative bias, as Al says, although here the DC grid-circuit resistance is already high because of the AGC feed. So if grid current is drawn, this is going to increase the bias anyway, and charge up the AGC decoupling capacitors negatively.
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