Definition
A vacuum tube with five grids used in older superheterodyne radio receivers to perform frequency conversion. It combines the functions of a local oscillator and a mixer in a single tube, taking the incoming radio-frequency signal and an internally generated oscillator signal and producing a fixed intermediate frequency that the receiver's later stages can amplify and demodulate.
Plain English
A type of radio tube used in older aircraft receivers that mixes the incoming signal with a steady tone generated inside the tube, producing a single fixed frequency the rest of the radio can work with easily.
Context Anchor
Seen in older aircraft radio equipment descriptions, avionics maintenance manuals, and discussions of vacuum-tube radio receivers.
Derivation
From Greek 'penta' meaning five, and 'grid,' referring to the wire mesh elements inside a vacuum tube. So 'pentagrid' simply means a tube with five grids — each grid handling part of the signal-mixing job.
Why Pilots Care
A pilot is unlikely to operate a pentagrid converter directly, but the term can appear when reading about older aircraft radios or maintenance of vintage avionics.
Intuition Check
Do not read “converter” here as a power converter or adapter. In this term, it means a radio part that changes a signal from one frequency to another.
Example Sentence 1
The avionics technician traced the loss of reception to a failed pentagrid converter in the receiver's front end.
Example Sentence 2
Restoring the old ADF unit required installing a new pentagrid converter because the original tube had failed.