Ground Loop? Bad Wiring?

[Alonzo]

Hi Everybody,
Thanks for all the suggestions.  Sorry I haven't responded to the posts, the "F##kUps" in D.C. put us on furlough, and there's a thousand things to do to get a project stopped so I've been a little busy.  I tried to recreate my spark condition, but after all the rebuilding and changing around I couldn't.  Here's where it's at, still have hum when all the items are connected, individual amp to speakers " no hum".  Here's a pic of the quickie, batteries in the back, RCA's out to 45 or el84 on one side, RCA's out to Crack on the other.  White wire running across plate goes to power block ground post, red wire to Crack ground.  I think there's a problem with my Quickie switch, looking for a replacement but I don't see how that would induce hum into the amps.

[corndog71]

Have you tried it without all of those extra ground wires?

[Grainger49]

Well, I see the problem with the system, it is upside down!

Sometimes lots of grounds create hum problems.  I would take all the ground wires off.  Remove the system interconnects and verify chassis and signal grounds are tied to the power cord ground.  Easy peasy just grab your meter and check that it is less than 1/2 ohm from the power cord ground to both the chassis and RCA plug shells.

Then plug in each component, turn it on and verify that from ground at the wall or the power conditioner to the chassis you have zero volts.  You want no floating voltage on the component chassis. 

Now get a cold beer.  Slowly plug in each component into the system and see when the hum comes back. 

Since the hum is in the 45 amp when the Crack is also attached, I assume a Y-Adapter of sorts, the interaction of the two might be the answer.  One or the other doesn't have as good a ground in the signal path as the other.  My ST-70 had a 10 Ohm resistor between the RCA jack ground and the common/ground.  It caused problems with the FP III.