"Finally, you need to ground your OT secondary. I'm often surprised to see folks online suggesting otherwise. First and foremost, an ungrounded OT secondary means your speaker lines will rise to B+ in the event of an OT fault. That's a safety issue and is not acceptable. Second, if your amp has NFB the OT secondary must be grounded at your power amp star point for proper ground referencing. Finally, even if you don't have NFB an ungrounded OT secondary will be capacitively coupled to the primary and can thus have a high AC voltage swing. This high AC voltage swing can make its way back to the input path and cause squeal. And, the OT core should be grounded to the chassis via the mounting bolts."
+1 I remember not grounding the OT on an early build of mine - not only did it have a bizarre squeal that seemed related to it, but I actually got a hearty shock when I touched a switch on the attenuator it was plugged into (my other hand on the guitar strings). A common ground reference is a good thing.
FWIW, I've found that it makes pretty much zero difference where you ground the speaker jack, just go for the shortest wire run.
Life is a tale told by an idiot -- full of sound and fury, signifying nothing.
I misunderstood Aiken's article as well a while back, and didn't ground the speaker jack of my non-NFB amp. While trouble shooting the amp, which was not making any sound, I hit the strings open and went to wiggle the speaker cable. I got a nasty shock from the speaker cable's sleeve. Luckily I was not touching the strings. Sure enough, I measured a very high AC voltage on the sleeve.
I have no idea why Aiken's article says otherwise, or perhaps it's too ambiguous or phrased poorly.
If you are using switchcraft style open jacks and it is bolted to the chassis, then the common wire is grounded to the chassis.
I think what Aiken was saying is that some amps ran the common wire to the chassis near the OT.
He says not to do that but instead run it to the jack and solder it to the sleeve lug.
The part about the NFB is to run another wire from that sleeve lug and ground it where the cathodes of the phase inverter ground.
Note he says that not much current will flow through that wire, it is more a signal ground than anything.
