Detector Diode Circuits

grid leak detector?

hi, i wanna build a simple AM valve/tube radio.
the grid leak detector has come to my attention.
i am referring to the resistor and capacitor in the grid circuit:

http://www.bolis.com/amillar/pg/rdamh/fig048.png

what is the resistor for?
as i understand it, the electrons floating by the grid sometimes hit it, naturally pushing it negative with respect to the cathode.
how far negative does it go? to the cut-off point of the valve, or just to the point where the grid stops acting like a diode with the cathode?

The resistor will slowly discharge the capacitor. If there were no resistor a peak charge would build up on the capacitor and cut off the tube.

The electrons flow from the cathode to the plate. The cathode is red hot and has lots of free electrons. The electrons will flow to the positive plate. With a negative charge on the grid the electrons have to squeeze by to get to the plate. The more negative the voltage (on the grid with respect to the cathode) the more the electron flow is attenuated. Finally with enough negative voltage on the grid no electrons can get through (cut off).

What grid leak bias is, is when the grid is driven slightly positive by the signal. A small flow of electrons will be flow onto the grid. The grid is now less positive which is a bias that attenuates the flow of electrons to the plate. It is an on going process to keep bias on the grid. With sufficient drive the bias will reach cut off. One bad thing about grid leak bias is that when there is no signal to drive the grid you loose the bias and the plate current will go high, limited only by the resistance of the tickler coil and headset.

Hope this helps

D.bumstead

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What is the function and importance of the RC circuit at the output of the diode detector?

It's a low pass filter actually - allowing the modulation to pass but not the r.f. component.