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A tri-tet oscillator is a crystal-controlled vacuum tube electronic oscillator circuit. It is a type of electron-coupled oscillator (ECO), which uses a tetrode or pentode tube.
The tri-tet was introduced by James J. Lamb in the June 1933 issue of QST magazine.
In an ECO, The screen grid acts as a plate for the triode oscillator. Some of the electrons pass through the screen grid to the plate, causing a current to flow in the load. This effectively isolates the oscillator portion from the load, so that loading does not affect the operating frequency.
In the schematic shown, the C1/L1 tank circuit control the current through the crystal. It is normally tuned a point between the crystal's fundamental frequency and its first harmonic.
The C2/L2 tank circuit is tuned to the desired harmonic.
D+ acts as the "plate" voltage for the tetrode, and is slightly less than B+.
Tri-Tet in popular culture
Jean Shepherd occasionally mentioned the tri-tet oscillator in his writing and radio shows. It was used as the oscillator in one of the first amateur radio transmitters he used.