Deep Dive: G6A12
The correct answer is A: To reduce grid-to-plate capacitance. The primary purpose of a screen grid in a vacuum tube is to reduce grid-to-plate capacitance. The screen grid acts as an electrostatic shield between control grid and plate, reducing unwanted capacitance. For amateur radio operators, this improves tube performance. Understanding this helps when using tube amplifiers.
Why Other Answers Are Wrong
Option B: Incorrect. Screen grid doesn't directly increase efficiency - it reduces capacitance, which may improve performance, but efficiency increase isn't the primary purpose. Option C: Incorrect. Screen grid doesn't increase control grid resistance - it reduces capacitance, not resistance. Resistance isn't the issue. Option D: Incorrect. Screen grid doesn't decrease plate resistance - it reduces capacitance, not resistance. Plate resistance isn't affected.
Exam Tip
Screen grid purpose = reduce grid-to-plate capacitance. Think 'S'creen 'G'rid = 'S'hields 'G'rid from plate. Acts as electrostatic shield between control grid and plate, reducing unwanted capacitance. Not efficiency, not grid resistance, not plate resistance - just capacitance reduction.
Memory Aid
Screen grid purpose = reduce grid-to-plate capacitance. Think 'S'creen 'G'rid = 'S'hields 'G'rid. Acts as electrostatic shield between control grid and plate, reducing unwanted capacitance. Primary purpose of screen grid.
Real-World Example
A vacuum tube has a screen grid between the control grid and plate. The screen grid acts as an electrostatic shield, reducing the capacitance between control grid and plate. This prevents unwanted feedback and improves tube performance. Reducing grid-to-plate capacitance is the screen grid's primary purpose.
Source & Coverage
Question Pool: 2023-2027 Question Pool
Subelement: G6A
Reference: 2023-2027 Question Pool · G6 - Circuit Components
Key Concepts
Verified Content
Question from the official FCC General Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the G6A topic.