Puzzled by a gray wire inside of the Hammond 369EX transformer

ipetruk · 2173

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Offline ipetruk

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Hi dear Bottlehead forum,

I assume it is ok to discuss non BH kits in this topic. I am trying to make 4S preamp, which is not a kit, just a schematics.

I've got a Hammond 369EX transformer for the power supply. It is a multi-tap transformer for many voltages, half of which I don't need. Unlike PT transformers in the Bottlehead kits it has wires, not lugs coming out if it. I asked forums and chats what to do with those, pretty much everyone told me to trim them to minimal length that allows future reuse and hide them in a bell, isolating them with heat shrink.

So I did it. However, when I took it apart I notices a gray wire inside of the transformer. One end goes inside of the transformer, another end is soldered to a star lockwasher on one of the bolts. All inside of the transformer. The datasheet for 369EX calls it "electrostatic shield" wire, no explanation given. https://www.hammfg.com/files/parts/pdf/369EX.pdf

What puzzles me is that I don't understand if it is expected to be grounded. Wire does connect to the bolt, but the transformer is painted black and it's paint is not conducting. The steel plates making a magnet (core?) are kinda scratched where bolts are placed, including the one for the grey wire. But it all of this does not look reliable: who guarantees that the lockwasher inside is reliably conducting to the bold and the bell.

Also how do I mount it to the chassis in that case? 369EX is a "sideways" transformer so bolts are horizontal and legs are separate. See the photo attached for how it looks and for the lockwashed bolt at the right bottom that is internally connected to the "grey wire".

Do I just scrape a leg or use a star lockwasher on my legs when mounting the transformer to the chassis? It is interesting, Bottlehead kits use plastic lockwashers for the transformer, so it was never clear to me in BH kits if there is an intention for the bell to be electrically connected to the chassis? What are the general rules about grounding transformers, and why plastic lockwashers are used?

Roon -> Raspberry 4 -> Khadas Tone in Bottlehead Crack+Speedball -> detachable cable modded DT880


Offline Doc B.

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Big topic. Hammond should be asked, since they made the transformer.

You want to put it back the way it was. The wire connects an internal shield around/between the windings to the transformer frame. When the transformer is mounted to the chassis the frame will then be connected to the chassis, thus the internal shield connection becomes connected with the chassis ground plane.

Regarding our transformers, they do indeed get isolated from the chassis at their mounting point. However - very important - when they are mounted to the chassis they have a lug attached to a mounting screw (and thus also the frame) that is  attached by drain wire to the chassis at the safety/chassis ground junction. So they are connected to the chassis as well. This is for safety and must not be left off. We do it this way to avoid ground loop eddy currents developing between the inductors, which can add noise.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.