History of the company that invented and patented the overtone crystal, etch-to-frequency and variable crystals. Source for ads, engineering bulletins, patents and manuals.

FT243 information compiled by Paul Carreiro, N6EV.

A handy historical chart of old crystal holders with dimensions, and suppliers.

WB6SZS, Eugene Rippen's comprehensive chart for crystal use on the HF, VHF and UHF bands.

From N6EV. Tube oscillator schematics, and discussion.

A useful technical reference page for Crystals.