Power Transformer connection to AC cord

[osing]

Hi,


I am going to be building a tweed Bassman amp and have a quick question about the power cord. I can see two ways to connect it. The first one below follows what Doug shows in his layout and involves connecting the neutral wire from the power cord to one of the transformer primaries (presumably by twisting the wires together, soldering them and adding some heat shrink). The hot wire goes to the fuse, then to the power switch, and finally to the other PT primary.


In the second image below, I have connected the hot wire to the fuse and then to one of the PT primaries. The neutral wire and second PT primary wire are connected to the power switch. This would allow all wires to be connected to lugs rather than directly twisting wires together.


Is there any reason not to go with the second option?

[eleventeen]

Yes, there are two reasons, 1 theoretical (eg; rule) based, 1 practical.

We prefer not to insert the switch in the neutral. (that's the "rule")

Why not? Because suppose (A) your fuse is good, (B) the amp is plugged in to the AC outlet, (C) the chassis out of the cabinet and (D) the power switch is off.

That is not an unimaginable set of circumstances. When you decide something is wrong that causes you to remove the amp from cabinet the first time, you probably unplug it from the wall. Fair enough. Now, you are "working on" the amp and it is on your bench. And plugged in. And, you do something (eg; change a part) and while doing so, you probably just flip the power switch.

However, should you have the switch in the neutral, the hot of the AC line now travels through the fuse and through the primary winding of the PT, where it does nothing because no current is flowing. However, it surely energizes that convenient terminal you have soldered it to, and that is what the hazard is: Under bench conditions, you have a terminal that is full AC hot and is thus a shock hazard. It is that way when the amp is shut off.

In the same way that we would like a blown fuse to BE IN the hot and if it blows, to remove the hot from as many internal points as we can....we would like the switch to remove the hot from as many points as we can. Ergo, switch goes in the hot.

Obviously, the switch functions identically if placed in the neutral, and, even if you do things the reco'ed way, you could say that if the fuse is NOT blown then there is still a potential hot terminal even if the switch is properly placed in the hot: On the input side of the switch. But if you're asking.....the preference is to place the switch in the hot.

[osing]

Eleventeen, thanks for your detailed response! I think I follow, but I have one follow-up question. In the conventional scenario (my first drawing), with a working fuse and the switch off, both the fuse lugs and the one lug of the switch are energized when the amp is plugged in. Isn't that exactly the same in the case of my second drawing, i.e. the same lugs are energized, but no other ones? The only difference I see is that the two primary wires are hot so if they were damaged (e.g. stripped insulation) you might have some added risk of getting shocked. Either way, I will go with the conventional method, but I want to make sure I fully grasp this and am not missing something.

[eleventeen]

Yes, you are correct. The number of terminals exposed is ultimately the same....that is, until you take further measures. 


What I like to do is: At Home Depot, you can buy clear plastic tubing, 1/2" ID inner diameter. It fits perfectly over a fuse post. Or, if you have a piece of fat multiconductor cable, some of those fit over the fusepost nicely. Or, heatshrink would be the truly proper way to go, but I kind of like the clear tubing as it is more rugged, and the threads really hold it well. I use jacketing stripped from various cables to insulate such thing as switch terminals. Still like that better than heatshrink because it can be removed and replaced and has more gripping elasticity. And it is thicker than heatshrink. Another use for jacketing is (save those cuttings, kids!) if you have two fattish wires you are trying to solder to a terminal, I like to pre-solder and feed those wires thru the tubing, because if you carefully get the two wires even with each other in terms of how much of THEIR insulation is stripped off, it makes the lead-in to the soldered terminal much neater. By the way, CAT5 cable is useful for this, the type that is used in conduit, and most installations throw out tens of feet of it. The jacketing of that cable is very useful, all over the place. And free.


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So if you cover up the whole fusepost AND the terminals on the power switch, then leaving the amp plugged in to the AC Line while you work on it, there is no way you can shocked by the line. There are plenty of ways you can shocked other ways, let there be no doubt.


The thing about having the hot "sneak" through the transformer primary (which occurs when you place the switch in the neutral AC feed, is just that---it's sneaky. Yes, if you insulate it, then there are no exposed anythings.


But again....if you ask...."best practice" is to place the sw in the hot lead!

[osing]

Very cool idea, and thanks again for your help!

[eleventeen]

And by the way---the jacketing stripped from Belden (or equiv) 8451 or 8422 which is twinax, foil shield, is magnificent for insulating splices between individual 20 or 22 gauge wires. Really works well, grabs a solder joint very well and does not slip off. Yet it can be slipped off and replaced if you want to. I again like it better than heatshrink because it's thicker and thus more rugged, and it's free. Oh, and doesn't require any heat treatment. Keep those 4 and 6" snippets of wire you throw out! I have splices I made 30 years ago with that jacketing which are perfectly good today.