| Electronics > Projects, Designs, and Technical Stuff |
| Why not grounded tetrode screen grid? |
| (1/1) |
| Circlotron:
We all know that compared with a triode, tetrode valves have an extra "screen" grid between the control grid and anode to reduce the anode/grid capacitance that causes RF feedback and instability. All of us also know that if the anode voltage dips below the screen grid voltage, secondary electrons can be drawn off the plate and onto the screen, causing a kink in the anode curves. Or something like that. Then the pentode was invented with a third, grounded grid between the anode and the screen grid that overcame this secondary emission problem. My question is - why was it necessary to have the screen grid positively charged? Could it not just be at ground potential like the third suppressor grid? Presumably it would still function as a screen grid but without secondary emission current and a kinked anode curve? Or couldn't it function as screen and suppressor at the same time by being at ground potential? The above is not rigorously accurate, but good enough to pose the question. |
| TimFox:
The cathode, control grid, and screen grid form a triode, where most of the current passes through the screen to the plate. In space-charge limited operation, the E field at the cathode determines the current. In a normal triode, the grid is much closer to the cathode than the plate, and the effect of the plate voltage is reduced (compared with a diode): this is where the amplification factor "mu" comes from. In a tetrode or pentode, the screen further shields the cathode from the plate voltage, and the cathode current is then determined by the screen and grid voltages as in a triode (relatively low mu). If you reduce the screen voltage to zero (with respect to cathode), the current would be reduced to almost zero. On page 3 of this data sheet http://tubedata.milbert.com/sheets/093/6/6SJ7.pdf you see the current with Ec1 = 0 (control grid, there is a typo on the graph legend) as a function of plate voltage with reduced screen voltage Ec2 down to 50 V: the current falls monotonically as you reduce the screen voltage. |
| Navigation |
| Message Index |