The AM Forum

Hopefully a simple question, although google searches are eluding me :).  After changing the line cords from 2 wire/prong to a grounded 3 lug line cord on older tube gear,  is it still necessary to use the ground lug on the chassis?

Thanks
Dave
KC3GMQ

I have a wide copper buss running along the back of my radio desk.  It's drilled and tapped for screws and every piece of equipment is connected to it via a short strap.  That way, I know by observation that everything is connected to the station ground and I don't have to worry about long circuitous paths throw the AC supply system.
My buss is a quarter inch thick and two inches wide, and it connects via copper strap to the outdoor ground system.  If buying copper buss seems expensive, a piece of schedule 80 copper water pipe from Home Desperate will tap for threads pretty well as long as you don't get too enthusiastic with the screwdriver later on.  Use wood screws to affix it to the desk for stability.
But I want to see my RF ground and not wonder.  I don't have "rf in the shack problems."

I have a similar set up now. I was looking to see if it was still necessary when you have the 3 prong grounded supply line. I'll keep them connected

Here in the NE, we can have some really bad storms and for that reason I use # 6 stranded wire routed to two separate ground rods. I have removed the insulation on the # 6 that goes directly to my BC-610 and the rest of the major equipment, like the antenna tuner and receiver is likewise is grounded to the same point using # 6 insulated wire. I left the insulation on where the wire routes through various equipment so I would not get any accidental contact on something I cannot see.  All of the smaller stuff like pre amps etc are grounded via the third prong.

 In the past before I had things grounded as they are now,every once in awhile I would be unscrewing a coax fitting and would absorb some unwanted voltage and that makes for a possible bruised limb. No more knee jerk reactions now!

Definitely still use the ground lug.

Electrical cord grounds are meant for 60Hz safety and are not optimal for RF. There are varying opinions on what makes for a good RF ground such as copper braid to cold water pipes, radiators, electrical boxes, structural steel (if any), random lengths of wires, etc, and avoiding multiple paths (ground loops) by using a "spoke" system of connections.

Basically, you can't have too much grounding...

Thanks

If you have ground rods connected to your station ground buss, you must connect your station ground system to your AC panel ground with a heavy copper line.

Yup, you is right on this account. I remember Nick KG2IR telling how they're grounded two skyscrapers together at Rockefeller Center. The other building was across the boulevard from 30 rock and they ran three sets of stranded cable each being about as thick as an average human wrist! This was done to put both buildings the same ground potential for all their TV and recordings that go on there both live, and canned. That was done to minimize hum dingers getting into their audio,  I may have distorted some of the facts on wire size because it was a long time ago when Nick told the story,   

Single point (star) grounding is the correct way, IMHO.

However, no matter what 'extra', rf, whatever ground you install, it all must be bonded to your electric service e panel ground.  Which is also bonded with your cold water and gas (in my jurisdiction).

The size of the wire you use for the run from your station ground buss to the rest of the AC service e ground system is dependant on your main service breaker.

Don't use braid.  Dumb idea.  The added inductance of the braid will raise whatever is connected to the SPG by whatever value of X the braid has at the frequency of interest.  Like the caps and coils lifting the grids of 3-500Z and other triodes.   Use strap.  Wide strap.  And if you don't or can't use strap, use big wire.

You should add a ground rod outside the station and run a wire from your AC panel through the rod and into your ground buss inside.  The wire should be continuous.

If you read the works of Henry Ott, which has been expanded by Jim Brown, K9YC, then I'm wrong.  They no longer believe in a single ground buss.  Instead, they are teaching ground your amp, and then from that, in parallel, run strap from device to device.  This keeps the braid from being called on to keep all pieces at the same potential.  Most people can't solder a connector correctly, so they end up with crap connectors. Which in turn elevate one chassis in potential from another.

Hope that helps.  You can Google the size ground wire you need, but the general rule is...

100A breaker, #8
125 to 150 breaker, #6
175 to 200 breaker, #4.

Your local AHJ may have different sizes or requirements, butwhwre I work is pretty anal, so you should be safe with that guideline.

--Shane
KD6VXI

Thank You --Shane KD6VXI