Odd wiring in tube ampmschematics

For a 3 wire power system (with grounded chassis) that is incorrect as you suspect.

For a two wire power then it is correct to have the switch on one side of the line and the fuse on the other as you don't know (unless the plug is polarized) which side is hot when plugged in.

John :-#)#

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Ahh, that makes sense. I bet a lot of the designs copied existing older circuits where possible. Thanks, Eric

There is that. I tend to replace line cords with those using polarized plugs, if possible, and then keep the fuse (if any) and switch on the 'hot' line.

General, I would not add a fuse to an AA5, but rather use a purpose-made fused box, plugging the radio into that. In such a way, the fuse level could be varied, and the hot will surely be fused in any case. '

The problem with converting old amps to 3-wires is a matter of hum-loops. Just don't do it. Polarizing the plug is fine. There is a very long explanation of why this is so, but "hum loop" should be enough.

But, at no time with a repaired/restored amp should power exist on the transformer or chassis with the power switch in the "OFF" position. Full Stop.

Peter Wieck Melrose Park, PA

You're right, that is obsolete practice from the days of unpolarised ungrounded power cords. I don't think it ever had any advantage from a safety perspective, it was just fractionally easier to terminate the mains cord onto a switch & a fuse.

NT

I am getting into this late.

When replacing the line cord on an older 2 wire cord ( no ground) with a

3 wire grounded cord, which is the perferred method ? Go from the hot wire to the switch or the fuse first, then to the other and transfromer , back to neutral ? Of course the grounded pin on the plug goes to the chassis and the 120 VAC is isolated from the chassis by the transformer.

Several of us were discussing this the other day, but none of us knew for sure which should come first, switch or fuse in the 3 wire 120 vac equipment.

It did seem that in the older ARRL Handbook the 2 wire cords had the fuse on one side and the switch on the other side of the transformer.

Fuse & switch should both be on the live side. Fuse should come first, switches do sometimes short. Wires to switches can also come adrift.

NT

A 2-pole switch would be the first thing I would add.

Maybe your wall plus is not respecting the norm.

So, what you call neutral might the the phase line.

To comply to safety rules, manufacturers must protect the phase line, not the neutral (VDE, UV, IEC, ANSI...).

Maybe your wall plug is not respecting the norm.

So, what you call neutral should be the phase line.

Seen from you, the phase should be on the right, neutral on the left (plug in the wall).

It's easy to check, take a voltmeter (scale 500V AC), take one end (barefoot), and put the other one in the plug (each holes, one by one). You have no risk ; if it is the phase line , the meter should move a little. Don't forget that if you put the meter in Amperes, you will reach the cieling.

To comply to safety rules, manufacturers must protect the phase line, not the neutral (VDE, UL, IEC, ANSI, ISO...). So your wall plug connexions might be inverted.

Almost all of those projects aren't done by engineers. They're done by people parroting one another.

Back in the "good old days" of two wire non-polarized line cords, the standard procedure was one side went to the fuse, if it had one, the other side went to the switch. There was no engineering behind that decision, it was strictly manufacturing, "We need one less tie point."

Look at it as a series loop. It doesn't matter the sequence that anything is connected in.

Three wire line cords: Hot, to fuse holder (The point at the far end of the fuse) then to the power switch. Not that you should rely on an open fuse to keep your fingers off the hot side of a line inside a chassis.

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I said with a voltmeter 500 VAC (some 1MOhms) in between you and the phase, The measure should show about 5-10 VAC if you don't come from the shower but barefoot.

The neutral is easy to check ; around 5-10VAC betweent neutral and ground (a water pipe for instance or a heater in case of collective heating).

If you are seeing an actual 5 - 10 VAC on the meter between neutral and ground in a household system, and the wiring is otherwise to-code (US NEC), something is wrong.

Peter Wieck Melrose Park, PA

Probably under sized wiring, or a bad connection somewhere along the neutral line. A couple of volts is normal due to the normal voltage drop in a conductor.

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Maybe so. But that is enough to play havoc with such unimportant items as pacemakers, not to mention what it might do to audio and video equipment.

Peter Wieck Melrose Park, PA

how would a couple of volts mains drop have an effect on pacemakers?

NT

round in a household system, and the wiring is otherwise to-code (US NEC), something is wrong. "

Not true. If one side is loaded heavily that's what happens. One house, win dow AC units, for example had 25 volts on the neutral because all the A/C u nits were on one side of the line.

In that house it was because the job wasn't planned right. It was code but a shit job. And nothing wrong.

This can also happen in some older houses when the main neutral coming in i s a smaller gauge than the hots. Even if not. If you put too much load on o ne side it happens.

Then if you blow the neutral fuse, which should not be there logically, it can throw 240 volts into everything on the lightly loaded side. Many houses around here are older. They did not anticipate all the load we have these days. I have had to change the lead in from the meter more than once, other wise I would have had to put in like a 50 amp main or the job wouldn't be c ode. I got them ready for the service upgrade to 100 amps. Actually 100 amp s is not all that much these days. Not enough if you have a hot tub/jacuzzi or an electric furnace. If you have both, double it to 200 amps. (then the y will probably put in a peak load meter to pick your pocket more effective ly)

It won't. It also should not affect any audio or other equipment that is designed right. The only POSSIBLE problem might be turntable hum but even then...

designed right. The only POSSIBLE problem might be turntable hum but even t hen...

All domestic mains current loads are balanced, ie live & neutral carry the same & opposite current, resulting in nearly zero magnetic field. Pacemaker s, like any life-critical medical equipment, are designed & tested to meet harsh real-world conditions & keep going.

NT

** I would opt for switch first, then fuse.

This is for the benefit of silly owners who like to swap back panel AC fuses while the item is plugged in - relying on the switch to isolate both ends contacts on the fuse holder.

Something that needs to be checked, cos only some fuse holders do this.