*Question G6A12:
What is the primary purpose of a screen grid in a vacuum tube?*  

*Correct Answer is  A: To reduce grid-to-plate capacitance*  

### AI Response: (may contain errors)  

The primary purpose of a screen grid in a vacuum tube is to reduce grid-to-plate capacitance. Here's why:

1. **Correct Answer: A: To reduce grid-to-plate capacitance**
   - The screen grid is placed between the control grid and the plate (anode) in a vacuum tube. Its main function is to shield the control grid from the plate, which reduces the capacitance between these two elements. This reduction in capacitance helps improve the tube's performance at higher frequencies by minimizing unwanted feedback and oscillations.

2. **Why the other choices are incorrect:**
   - **B: To increase efficiency**
     - While the screen grid can indirectly affect efficiency by improving the tube's performance, its primary purpose is not to increase efficiency. Efficiency improvements are more directly related to the tube's operating conditions and design.
   - **C: To increase the control grid resistance**
     - The screen grid does not affect the resistance of the control grid. The control grid resistance is determined by the grid's material and construction, not by the presence of a screen grid.
   - **D: To decrease plate resistance**
     - The screen grid does not directly decrease plate resistance. Plate resistance is a characteristic of the tube's design and operating conditions, and the screen grid's role is more about reducing capacitance rather than affecting resistance.

In summary, the screen grid's primary role is to reduce grid-to-plate capacitance, which is crucial for stable operation at higher frequencies.