Tri-tet oscillator
A tri-tet oscillator is a crystal-controlled vacuum tube oscillator. It is an electron-coupled oscillator (ECO), which uses a tetrode or pentode tube.
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.[1]
The tri-tet was introduced by James J. Lamb in the June 1933 issue of QST magazine.[2]
In the schematic shown, the L1/C1 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 L2/C2 tank circuit is tuned to the desired harmonic.
D+ acts as the "plate" voltage for the triode, 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.[3]
Some of the trials and tribulations of using this oscillator circuit are documented on the internet, including articles about damage to quartz crystals dating back many decades. Other oscillator designs are known to be far safer when using any crystals, but especially those newer crystals requiring less than 20 - 40 mA of drive current. Older crystals could sometimes tolerate 60 mA of current through them without damage but it was common practice to place a 60 mA pilot lamp in series to monitor current and acoid damage when using the TRI-TET oscillator designs.
References:
http://mysite.verizon.net/dpnewkirk/ej/tri-tet/
http://mysite.verizon.net/dpnewkirk/ej/boosted_pierce/
References
- ^ Sterling, Geroge E. 1940. The Radio Manual for Engineers, Inspectors, Students, Operators and Radio Fans. 3rd ed. New York: D. Van Nostrand Company, Inc.
- ^ Lamb, James J. “A More Stable Crystal Oscillator of High Harmonic Output.” QST June 1933: 30-32.
- ^ Shepherd, Jean (October 1980), "Some Guys Make It", 73 Magazine, Peterborough, NH