John Coleman ARS WA5BXO wrote:
If an RF chassis is a complete self contained rig, and has an analog modulator and microphone preamp built into it, then the modulator preamp circuitry should be on a separate sub assembly which is mounted via insulation and has only one ground connection to it. Inside the audio sub assembly there should be a ground buss which is connected to the sub assembly chassis at only one place. I generally will use some bare #12 wire and bend it so as to make small bridges across several insulated terminals spanning the bulk of the underside of the sub assembly. If I need a ground point such as a cathode resistor or grid leak resistor or one end of a shielded cable then I use the buss for the ground connection. In most preamp sub assemblies the single ground is at the microphone input. This way the microphone connector is mounted directly on the chassis sub assembly. The audio sub assembly is then mounted in such a way as to insure that its only ground is near or surrounding the microphone connection. In some cases it is necessary to make the ground point somewhere else. When this is the case the microphone connector should not be mounted on the chassis but on some insulating material and the ground connection for it should be made to the ground buss. To be safe, it is best to rectify and filter and resistor limit the 6.3 volt AC for the heaters of the audio preamps. Do this all outside of the audio preamp sub assembly. Then ground one side of the heaters to the ground buss. Although I have had good luck with a balanced AC and a filament hum bucking control it is just easier to do the DC filaments for a few tubes in the high gain preamps. I can't believe I typed up all the stuff!!
*This* is a homebrewer! Good job, John! Proud to know you as a 'friend'. -- -Geoff/W5OMR
