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Re: Screen Grid Q

10/24/2005 8:01 PM
Re: Screen Grid Q
If you can find the plate curves for a 6973 in an old RCA book, it shows what the screen cureent does. It's a dotted line "Ic2" taken with Ec1=0 and Ec2=250V. When the plate voltage goes below 50V or so, the screen current shoots up from 20mA or so at Ep=50V to 90mA at Ep=20V.  
I read somewhere that the spiral wires of the screen are supposed to be directly behind the spiral wires of the grid. The electrons, being negativly charged, are repelled by the grid wires and forced into the space between the wires (if the grid isn't so negative it won't let any past). Once they make it past the grid, they accelerate and most can't make the turn to hit the screen although the may deflect towards the screen. The electrons would tend to spread out anyway because they repel each other.  
When tubes are designed, many mechanical aspects of electrode spacing and construction are considered against their influence on electrical characteristics. In other words, some really smart guys figured out how to make the screen so it won't hog all the current.

Enzo Yes, the screen wires do hide behin... -- 10/24/2005 8:58 PM