Thirty (30) Machlett Laboratories (ML) 6697A Electron Tubes (10 each year for three years) to provide the final power amplification for the Army-Navy/Frequency Radio Transmit-31 (AN/FRT-31) Very Low F... Thirty (30) Machlett Laboratories (ML) 6697A Electron Tubes (10 each year for three years) to provide the final power amplification for the Army-Navy/Frequency Radio Transmit-31 (AN/FRT-31) Very Low Frequency (VLF) transmitter required by the Naval Computer and Telecommunications Area Master Station Atlantic (NCTAMS LANT) – Detachment Cutler.