1. A valve can contain more than one control grid . 2. The circuit that drives the control grid is usually referred to as a grid modulator. 3. A variation of the control grid is to produce the helix with a variable pitch. 4. As in the dynatron oscillator the control grid can be used to adjust the negative resistance. 5. This additional " control grid " modulates the current that flows between cathode and anode. 6. In the off situation the voltage on the control grid is negative with respect to the cathode. 7. Moreover, control grids are required in order to adjust the timing of the start of conduction. 8. A relatively small variation in voltage on the control grid causes a significantly large variation in anode current. 9. In fact, each tube in VFD is a triode vacuum tube because it also has a mesh control grid . 10. By placing the control grid closer to the filament / cathode relative to the anode, a greater amplification results. 
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