Definition
In vacuum-tube electronics, a C-battery is the dry-cell battery used to supply a small negative bias voltage to the control grid of a vacuum tube. This bias sets the tube's operating point so it amplifies signals correctly without distortion.
Plain English
A small battery whose only job is to hold the control grid of a vacuum tube at a steady negative voltage, so the tube works the way it should.
Context Anchor
Seen in older aircraft radio, electrical, and maintenance references that describe vacuum-tube equipment.
Derivation
The letter designations come from early radio practice. The A-battery heated the tube's filament, the B-battery supplied the high plate voltage, and the C-battery supplied the grid bias. The letters simply followed the order in which each power source was added to tube circuits.
Why Pilots Care
Correct C-battery voltage keeps tube avionics stable; low or reversed voltage produces distortion, weak signals, or complete failure in classic aircraft.
Intuition Check
C-battery here does not mean a common size-C flashlight battery. It means the C battery in the old A/B/C radio-battery system, used to supply a control voltage.
Example Sentence 1
The technician traced the distortion in the old receiver to a weak C-battery that was no longer holding the grid at the correct bias voltage.
Example Sentence 2
When the ADF receiver sounded distorted, the technician replaced the aging C-battery and restored normal operation.