For sale, this homemade Relay Control Box to be used to key your linear amplifier. It is recommended to use this device to activate in transmission mode your liner amplifier and to prevent damage on your transceiver. Will only work with amplifiers that requires a connection to ground to activate TX mode. With this device your transceiver will be completely isolated from your linear amplifier. Each unit is tested prior shipping, and you have one year warranty ...to honour warranty you must submit your original order detail. Have one of this myself using a FT-747GX and a Dentron GLA-1000...been working with no issues for the last 5 years. Note: Your radio must have a "PTT" output ...and your amplifier must have a "Relay or Key" input in order for this unit to operate