Installing 3 Conductor AC Cord In Old Amps,,,

[zozoe]

Good day all & hope all is well. I'm following everyone's good advice, getting smart & installing grounded 3 conductor cords into some old 50's P.A. amps. I've watched all the YouTube videos, and all of them deal with guitar amps w/polarity switches & 'death caps', & don't see any clear, confident or consistent info. Okay, Green is GROUND,, some videos show the White Neutral going directly to one of the trannies, and the Hot Black wire going to the power switch then on to the fuse, & then to the tranny. Then others show the white neutral going to the fuse & the black hot
to the On/Off,,, AND vice versa, w/white to the power switch, & black to fuse. These are old mono tube amps, & as I mentioned, no polarity switch or cap. I want want to be another toasted statistic, or fry anything.
Thanks in advance for your assistance!
BE WELL!!

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[toddgw]

My rule is to always route the 'Line' or black through the fuse, regardless of the switch. A fused neutral still leaves the AC voltage connected to the equipment if the fuse blows. And yes green should always be ground. Make sure to get yourself a solid connection to the chassis with your green wire. Either use a 200W or greater iron to solder directly to the chassis, or use a captive solder terminal under the power transformer mounting screw/nut.

[Colossal]

Zozoe,

Please check your Private Messages. Upper right-hand corner of your screen...

Thanks,
Colossal

[M Fowler]

Like this.

2 to 3 prong conversion bypass grnd switch.JPG

2 to 3 prong conversion.JPG

[ChopSauce]

A fused neutral still leaves the AC voltage connected to the equipment ...

That's it!

Please also note that the above layouts are no substitute to thinking about what you're doing along these lines: you don't want the live/line or any "high voltage" current to flow to ground through your body.
(which implies that you should "cut" it as soon as possible in the circuit)

[zozoe]

So, am I correct in that the white wire in the amp that was originally connected to the fuse holder, is attached to me white on my plug? The back that goes to the tranny from the power switch, is never touched, but it's the white to the fuse that I have to undo in order to make room for the new incoming back wire & link to the power, correct? I ain't touching anything until I get a definite yes/no.
BTW, there's no ground switch in my amps, which so many diagrams show & don't address my primitive 50's designs....
Thank you.

[Leo_Gnardo]

Some manufacturers, including highly respected Fender, often fused the neutral side of the AC feed. Not the smartest thing to do. In case of a fault that connected hot AC to chassis, the fuse would pop BUT hot AC remained as a hazard. When the first thing the hot side sees is the fuse then an open fuse disconnects power from the chassis unless the fault is with the AC power cord itself.

When you're adding a grounded AC cord to an old amp you have the opportunity to do things right, and I hope you will.

Also - connecting ground and neutral together in an amp is a bad idea, no matter that some manufacturer did it way back when. The only place ground and neutral go together is a single bonded connection within the AC power breaker box for the building, as far as I know. Experts are welcome to correct me here if necessary.

[M Fowler]

You can also wire the hot to the switch before the fuse so when off everything is dead.

The fuse holder is hot in the above layout but is how I wire mine.

Mark

[SoulFetish]

Some manufacturers, including highly respected Fender, often fused the neutral side of the AC feed. Not the smartest thing to do. In case of a fault that connected hot AC to chassis, the fuse would pop BUT hot AC remained as a hazard. When the first thing the hot side sees is the fuse then an open fuse disconnects power from the chassis unless the fault is with the AC power cord itself.

When you're adding a grounded AC cord to an old amp you have the opportunity to do things right, and I hope you will.

Also - connecting ground and neutral together in an amp is a bad idea, no matter that some manufacturer did it way back when. The only place ground and neutral go together is a single bonded connection within the AC power breaker box for the building, as far as I know. Experts are welcome to correct me here if necessary.

Not only is it a bad idea, I think it's illegal (but I'll double check). It essentially makes the equivalent of a bootleg ground. The green earth wire is not a current carrying conductor under normal operation.
Leo, you're essentially right about how Neutral is bonded to earth at the junction in residential US. But an electrician who knows electrical code can speak to the nuance better than I can. Plus, I believe in EU they have multiple earth points within a system.
I did want to mention that there is some really good info in this thread about updating older amps for safer, earthed mains wiring. But I would respectfully just take a little issue with one of the instructions/recommendations.
I would strongly advise against using one of the transformer mounting bolts to make your earth/chassis connection.
I know this is common practice among some builders, but it's probably one of the worst places to have it.
Mains transformers vibrate, some more than others, due to magnetostriction*. Over time can and sometimes does cause the nuts to back off the threads and can open the connection you are relying on as your safety harness. I do this kind of service on a lot of amps that come into our shop and I've seen it enough that I think it's a bad idea. Honestly, best practice is that the earth connection is given it's own designated fastener, not shared by any other serviceable part. Drill a small hole with enough clearance for a #8-10 stud. If it's done well, it can look really nice, be safe, be complaint, and you won't have to worry about it going forward.
If you want to solder it directly to the chassis, that can be perfectly safe as well when done correctly. Just my humble 2 cents on the subject.

*https://eng.libretexts.org/Bookshelves/ ... ostriction
*https://en.wikipedia.org/wiki/Magnetostriction

[ChopSauce]

... Neutral is bonded to earth at the junction in residential US. But an electrician who knows electrical code can speak to the nuance better than I can. Plus, I believe in EU they have multiple earth points within a system.

For (the sake of completion &) what's worth, I could read from a fairly trusted source (at forumG5) that the neutral - in France - actually is obtained through phase cancelation ...

[M Fowler]

Years ago in some SE builds I used the transformer bolt for ground but stopped that and have a dedicated solder ground lug for AC ground.

Mark