I've wrangled with this for awhile and I'm thinking the main safety issue here is to get the hot wire fused as soon as it's inside the amp. I'd like to know why it's not actually safer to fuse the hot and switch the neutral. My thinking is that live hot run to the power switch could be safer if it's a neutral.
I've posted a really nice explanation of what R.G. Keen (
www.geofex.com) has to say about line safety below. He also touches upon having the fuse and power switch on different lines, which is what Dumble did and says it's a very bad idea.
"In the USA at least, AC power comes off a large transformer nominally out on a power pole somewhere. One of the secondaries of that transformer is (usually) centertapped, and the center tap is grounded to the real, no fooling dirt in the hole the pole is standing in by a copper wire which runs down the pole to the bottom of the pole. This is a good ground, unless somebody has been playing Paul Bunyan and cut the copper wire, or stolen it to resell to scrap dealers. Three wires come to your house: Ground (that centertap) and the two hot sides of the winding. Each side is 120Vac nominal in the USA, and they're out of phase.
At your power entry panel, there is another ground, this one going to a metal pole driven into the ground. This is "safety ground", and is carried into your power distribution box. In the power distribution box, incoming ground is tied to safety ground, and this junction is the source of all the "white wire" or "neutral" wires in the premises. The two (usually black, to remind us of death. No, really.)wires become the two phases of power in most houses. Each wire feeds a hopefully equal batch of breakers in series with the actual AC sockets, light switches, etc..
At the wall socket: Any color other than white, green, or bare copper must be assumed to be hot, and must also be assumed to be live unless the breaker feeding that socket is PROVEN to be set off by meter reading the wires. In the bad old days, you had only white and black, or just two blacks. With no particular way to tell between them, you might as well open either line, right? Wrong, as we'll see. It was simpler to wire amps if you had a power switch to wire to on one incoming AC wire and a fuse holder on the other side, so it became common to put the switch in one side of the AC line and the fuse in the other.
This was a bad idea. If you flip the switch to your amp, and nothing happens, what's wrong? It could be the fuse blown, right? So you take out the fuse and in replacing it touch a grounded metal anything nearby, and your last thoughts are of a smell of burning flesh. The fuse in the neutral line left the switch on in the hot line, and when you touched the hot line and a local earth ground, you completed the circuit and died. Or you are very careful and turn the power switch off and then start replacing the fuse. In replacing it you touch a grounded metal anything nearby, and your last thoughts are of a smell of burning flesh. The power switch was in the neutral line, you touched the hot side of the fuse holder when doing the replacement.
The safe way to do this is to always put the fuse in the hot wire, not neutral. At least then your widow can sue the electrician who wired the socked backwards.
Safer is to put the fuse in the hot side and to use a DPST switch that breaks both sides of the AC line. Then your widow can't sue the electrician because it was you who didn't turn off the switch. Even better, if you ever replace the power cord on an amp, put in an IEC cord disconnect receptacle with safety fuse tray. Now you can just pull the plug out of the back of the amp."
It's a nice sounding tube.
