The vacuum-relay is mounted on a rectangle of c.16-gauge aluminum. A length of c.12mm by c.12mm [1/2" by 1/2"] aluminum right-angle stock is bolted to the bottom of the rectangle to form a mounting bracket&emdash;or, if a metal brake is available, a right angle mounting bracket can be fabricated. The RF reed-relay is mounted next to the vacuum-relay using a dab of silicone rubber. You may want to make a drawing of the pin-out numbers on the relay before you glue it down. Be sure to observe the coil polarity. The coil terminals are 3 and 5. Check indicated polarity. If the polarity is reversed, the relay will not work.
To reduce acoustic noise, the vacuum-relay is mounted without the use of nuts. To provide side clearance, the relay mounting hole is made 2mm to 3mm larger than the threaded mounting shaft on the relay. The vacuum-relay is shock-mounted with 3 "pillows" of silicone-rubber. I prefer the red-colored high-temperature General Electric Co. silicone-rubber adhesive-sealant.
It is important to position the relay so that no metal to metal contact occurs between the relay and the aluminum mounting plate. If contact is made, there will be an acoustic path between the relay and the chassis of the amplifier - like the sound bridge on a violin - and the chassis will act as a sounding board. To keep the vacuum-relay in the correct position while the silicone-rubber pillows are curing, three cardboard rectangular spacers are temporarily rubber-cemented around the mounting hole. The cardboard rectangles are approximately 1mm to 1.5mm thick, 4mm wide and 25mm long. The 3 rectangles of cardboard are spaced 120 degrees apart; each pointing at the center of the mounting hole like the spokes of a wheel. A few mm of each cardboard rectangle hangs over the edge of the mounting hole so that when each relay is pushed into the mounting hole, the protruding few mm of cardboard will be bent over and down. This insures that the relay will not touch metal while the silicone-rubber cures.
Silicone-rubber will adhere well to most materials IF the surface is prepared properly. The best surface-conditioning material I have found is the silicone-rubber itself. If the surface is greasy, first use a no-residue degreaser such as TCE, acetone, MEK, Freon™&endash;TF or ethanol ["Everclear"]. Next, apply a dab of silicone-rubber to a small, clean cloth and forcefully rub a thin film of silicone-rubber into all of the surfaces that you want to bond together. This information is not in GE's Official Directions. The bonding silicone-rubber should then be applied immediately -- before the conditioned surfaces start to cure. 3 dabs of silicone-rubber about the size of green-peas are applied before the relay is inserted into the mounting hole A small amount of silicone-rubber will do the job; excess silicone-rubber will enhance sound conduction to the mounting plate. No silicone-rubber should touch the cardboard spacers. The assembly should then be set aside for 48 to 72 hours of undisturbed curing. After curing, the cardboard spacers are removed.
The metal base of the vacuum-relay is electrically grounded to the aluminum mounting plate. Remove some of the paint from the relay flange.Solder a c.14mm long, 3mm wide, flexible "S"-shaped strip of c.0.1mm thick [4-mil] copper ribbon to the flange. Connect the ribbon to a ground lug on the mounting plate. Use a large soldering iron and be quick to avoid overheating the relay's ceramic to metal seals.
The mounted relay should be able to move slightly in its mounting hole without touching the metal mounting plate.
WIRING THE RELAYS
A vacuum relay's coil terminals can be easily broken off by sudden impact or too much stress. The wires that connect to these terminals should be flexible. 26 to 22 gauge stranded hookup wire is satisfactory. The RF terminals are wired with c.0.1mm thick copper-foil ribbons, c.3mm in width. No direct connection to either relay's RF terminals should be made with stiff wires. . If a connection is to be made between an RF terminal and a stiff wire, a U {or S}-shaped 3mm by c.20mm flexible bridge of copper ribbon should be soldered between the stiff wire and the relay terminal to reduce sound conduction and stress on the relay.
All of this may sound like a lot of trouble to go to, but the resulting quietness IS worth the effort.
Rich, AG6K