Ground Loops in AB763 Deluxe

[mikepukmel]

I was digging for images of actual vintage blackface, mid 60's, era AB763 or close, Deluxe or Deluxe REverb amps. With all of the discussion of ground loops, I noticed that these amps all had a piece of heavy buss wire leading from the 'self bias' cap resistor connections, (preamp tubes and tremolo tubes) off the circuit board, to the thin brass plate that is mounted under the pots.

There are at least 3 or 4 of these connections.

Also, there is a black wire leading from the cap board on top of the chassis, through a grommet, inside the chassis, that gets grounded by soldering directly to a blob of solder on the chassis near the power transformer. Also, there is another direct solder connection to the chassis, of two wires leading from the power transformer.

OK, so I don't know: won't this setup be prone to ground loops, since there are (5?) several places where a separate ground wire is connected to the chassis?

The opposite question: the location where the bus wires, and trans wires are connected, there are not a lot of other wires in that area, so I don't see any parallel wiring that might induce a current and cause a ground loop.

But (learned something new last week) I read about eddy currents (especially in aluminum chassis) that could be induced just about anyplace, e.g. around tubes and esp around transformers and chokes

OK obviously these amps are frickin' awesome, and most of us would love to have one, but why didn't they have more problems with ground loops?

[Leo_Gnardo]

why didn't they have more problems with ground loops?

Although Fender's grounding seems to be scattershot, there is a madness to the method. One important thing they did was to bring the ground connection for the lowest voltage filters to the ground bus bar behind the volume & tone controls.

Our fellow MEFster Randall created a Deluxe build a couple years ago. He used a piece of copper pipe as a ground bus. PAGING MISTER RANDALL ! ! ! Could you show us a picture of your Deluxe interior please? Short of that we may be able to search it out.

A cautionary story: Randall first built his project on black cardboard, the regular Fender stuff. Then found to his dismay it was loaded with unexpected problems: the black pigment is carbon black and somewhat conductive. Rebuilt on turret board, glass/epoxy, and it came out splendid the second time. I'm sure you'll want to take note of that.

[pdf64]

Have a good read of The Valve Wizard
The vintage Fender arrangement can work amazingly well; I think that in the terminology of Merlin's piece, it's kinda analogous to a multiple star ground.

[Randall]

Here is a shot of the DRRI I gutted and rebuilt. Although not copper pipe, I used a piece of the largest gauge wire available at Home Depot, I think it is 6 gauge. I rolled it on a hard floor under a short piece of 2 x 4 while tapping it with a hammer until it was straight as an arrow. Then I flew it as shown, attaching all the preamp grounds. I grounded it to a lug next to the input jacks. This amp is so quiet, I cannot sometimes tell if it's on with nothing plugged in.

[mikepukmel]

Here is a shot of the DRRI I gutted and rebuilt. Although not copper pipe, I used a piece of the largest gauge wire available at Home Depot, I think it is 6 gauge. I rolled it on a hard floor under a short piece of 2 x 4 while tapping it with a hammer until it was straight as an arrow. Then I flew it as shown, attaching all the preamp grounds. I grounded it to a lug next to the input jacks. This amp is so quiet, I cannot sometimes tell if it's on with nothing plugged in.

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Thanks, beautiful wiring job, nice build!