Which of the following indicates a defective vacuum tube in a transmitter?

• A. Gas blow within the tube
• B. No light from the filament
• C. Excessive plate current
• D. All of the above

Answer: (D)

A defective vacuum tube can manifest in several ways, and all the options presented are valid indicators of a malfunctioning tube.

Gas blow within the tube indicates that there is a gas leak or the presence of uncontrolled gases inside the tube. This condition disrupts the normal vacuum environment necessary for the tube to operate effectively, potentially leading to failure.

No light from the filament suggests that the filament, which heats up to emit electrons, is either burned out or experiencing an electrical failure. Without this heating, the tube cannot function, as it relies on the emission of electrons from the filament for its operation.

Excessive plate current can signal that the tube isn't regulating current properly, possibly due to failures in internal components or a breakdown in the vacuum. This condition can lead to overheating and further damage to the tube and associated circuitry.

Since each of these indicators signifies an issue with the vacuum tube, it is clear that all of them collectively point to a defective tube in a transmitter.