Ground loop troubleshooting [solved]

[Loquah]

I've just finished my SEX and everything tests well, but there's a nasty hum in the left channel which I think is a ground loop. It's unaffected by volume.

I anodised my chassis plate so chassis connections are inert unless filed back. I filed back the main earth connection, but no other spots.

2 questions:

1. Is there any other connection that relies on chassis earthing?
2. Other than manually earthing every ground in the circuit by soldering to the bare chassis, is there a way to test and isolate the source of a ground loop?

[fullheadofnothing]

First off, I'm not the most tech savvy person on this board, so even though I work at Bottlehead, take my suggestions with a grain of salt. On the other hand, I'm up and reading the forum at 10:30 on a Saturday...

Looking over the manual quickly, I think that several of the center positions of the terminal strips MAY be intended to be grounded to the chassis (terms 3, 13, 23), with the ground buss building out from there. The resistance checks should point out anything that is not grounded, but if you are attaching your probe to a floating ground (term 23, in this case) that wouldn't necessarily work. What happens if you read resistance between 23 and the chassis ground?

You mentioned that the hum is in one channel - have you swapped tubes to verify that it is not a tube issue (I know some 6DN7s can have a bit of a hum)? What are you plugging into your S.E.X.? I hear a lot more hum on my Grados than my AKGs or my speakers (signal-to-noise can be improved by using a lower impedance tap, fyi). Furthermore it could be source or cable related; some general system background would be helpful.

Edit-to-add:
Is this the same build as the one that has an un-draining power supply?

[Loquah]

Thanks, Joshua.

Different kit from the psycho zapper thread.

I tested everything using 23 so I'll go and check with a chassis or E ground.

Issue occurred through 2 different sets of low impedance earphones with and without a source connected.

I'll report back soon. Thanks for your comments.

[Loquah]

Turns out it is a faulty tube. Now to find a replacement.