Vacuum tube indicator in classic radios: The EM84 tube in a radio receiver is used as a _____ device.

Introduction / Context:
Many vintage radios used a special display tube to show tuning quality. The EM84 is a classic example: a bar-graph style tuning indicator that glows more or less depending on signal strength or AGC voltage.

Given Data / Assumptions:

• EM84 (6E3P) is a vacuum indicator tube, not a power or small-signal amplifier.
• It responds to the receiver's control/AGC voltage.

Concept / Approach:

The EM84 produces a variable illuminated display proportional to the control voltage, guiding the user to the point of maximum signal during tuning—hence the term 'magic eye.'

Step-by-Step Solution:

Verification / Alternative check:

Service manuals identify EM84 explicitly as a tuning indicator tube, distinct from audio or RF amplification stages.

Why Other Options Are Wrong:

• Audio/RF amplifier and rectifier roles are performed by other tubes/semiconductors.
• EM84 is not a frequency converter.

Common Pitfalls:

• Assuming every tube is an amplifier or rectifier; some are indicators.

Final Answer:

‘‘Magic eye’’ tuning indicator (visual signal-strength display)