Abstract: Diamonds are good thermal conductors and also good insulators. They are thus exceptionally useful as heat-removing supports for small microwave circuit elements which must be electrically insulated from their ultimate heat sink. Bonding diamonds to metallic parts has been done using active metals such as titanium and zirconium in conjunction with or as ingredients in solder alloys, which then are able to wet the diamond surface and bond to it.The invention comprises a method of diffusion bonding to diamonds without a molten solder. This solves the problem of contaminating the open diamond surfaces with braze materials which produce electrical leakage. Also the bonded area is precisely limited so that the electrical properties of the circuit are not disturbed.A tiny slow-wave circuit for a traveling-wave tube has been supported by a linear array of diamonds bonded to it and to the surrounding heat-sink barrel of the tube.