JMI Vox amp mains wiring: live to PT common?

[Interstellareye]

1EEF5A56-2B26-4D78-90EB-A90689C99E48.jpeg

I’ve been assembling an AC15 clone using the 1960 schematic, and Stephen Grosvenor’s Service Engineer’s Guide to the Vox AC30.

I’ve noticed in several JMI Vox schematics that the live mains lead is fed to the PT Primary common (post fuse and switch), and the Neutral mains lead is fed to the voltage selector switch and onto the PT primary voltage input.

As I understand it, there’s no practical difference between mains live and neutral, but it seems to be a convention in every other amp schematic to send the live lead to PT Primary voltage input, and neutral lead to the Primary Common.

So why does Vox do the reverse? Is there any reason for me to follow or not follow the Vox schematic for this mains wiring?

[ChopSauce]

Puzzling! I see no reason, either.

Is it the same for other Vox amps of the same era (or may we suspect that it is a mistake at some point - especially for the live being switched to a node labelled "0 volt ...")?

Nice project by the way and welcome to the forum...

[Interstellareye]

I’ve seen this wiring on a number of JMI Vox schematics and in photos of actual amps. But strangely, not all of them. Here is the power section from the AC30:

7B390275-A080-4238-AD57-C68FA0009F99.jpeg

Sometimes the fuse is on the Live wire, sometimes on the Neutral.

Most all JMI Vox schematics depict a DPST mains switch, with both L and N switching, but in AC15 photos (as pictured below) I’ve noticed SPST, switching just the L wire, then sending L to PT Primary 0v. The N wire goes directly to the voltage selector switch and thus to the PT Primary voltage input:

4244BAA3-E64A-4661-A145-57DBF8C08901.jpeg

A88079FF-7905-4DB1-86EF-60AFBA61204B.jpeg

You can see the same arrangement in this AC30 photo where the N black wire goes directly to the voltage selector switch and the red L wire goes to the Fuse holder, then to the SPST mains switch, and returns to PT with a black wire (presumably to Primary 0v).

4B7B4E01-B24A-499C-AD9B-AF868AAA46F6.jpeg

[thetragichero]

without a polarized plug there's a 50/50 chance you get line to the fuse when you plug it in anyway. wire to modern safety standards and keep it moving

[R.G.]

Actually, there is a practical difference between line and neutral, at least in the USA. Neutral is bonded to earth-rod ground in the distribution panel, so it's voltage everywhere along the wire is different from actual earth ground only by the I*R voltage from loading currents. Even with a substantial load current, this is only a few volts. Other countries may have different setups, though.
In modern practice in the US, it makes sense to use only the neutral as an AC mains wiring common, and put fuses, selectors, etc on the line/live side. As pointed out, there is a 50/50 chance of getting this backward in a two prong mains cord setup; and as mentioned, best practice would be to wire it to modern standards and move on.
I know from experience that the wiring you show in the schematic would not pass present safety standards. You want the fuse to be the first thing that the hot/line/live wire touches as it enters the chassis. Next should be the switch. Breaking both line and neutral for power switching is a really good idea, in case the socket is mis-wired, but it's not mandatory. After that, any voltage selectors, indicators, and so on, and the power transformer.
Safety ground should go directly from the power entry to a ground lug/bolt on the chassis, and it should be long enough so that if the line cord is pulled out of the chassis by force, the safety wire will break last.

[pdf64]

without a polarized plug there's a 50/50 chance you get line to the fuse when you plug it in anyway. wire to modern safety standards and keep it moving

UK wall outlets have been polarised and earthed even back to the 50s, mandatory British Standards covering electrical equipment would almost certainly specified to fuse the live feed, rather than the neutral.

... You want the fuse to be the first thing that the hot/line/live wire touches as it enters the chassis. Next should be the switch...

Do you happen to know if that's recommended or even mandated in any national / international standards, eg EN60335 and related docs.
Having retired, I don't have access to that stuff anymore.

[Interstellareye]

I’ve been discussing this same topic (Line/Neutral wiring on JMI Vox) on another forum, and I still haven’t come up with any decisive reason why Vox would have wired the Line (after fuse and switch) going to PT common/0v and Neutral going to PT Primary voltage input.

But the consensus seems to be that it won’t make any practical difference if I wire it “conventionally” with Line to PT Primary, and Neutral to PT Common/0v, and if anything, it’s safer this way.

So, that’s what I’m going with. I already figured I’d do it this way, but I just wanted to see if anyone had another opinion, or if anyone knew why Vox wired Mains the way they did. The only answers I’ve found is that they did it this way because it doesn’t matter, or because it was more expedient to the layout.

Someone else mentioned that early Fender’s also wired Mains this way. So perhaps it was just the conventional wiring for amps?

[SoulFetish]

In North America, the neutral conductor is the designated load/current return conductor. Furthermore, both neutral and earth conductors must be able to carry fault currents, with their gauge rated accordingly. For this reason, in North America, Live and Neutral are not interchangeable and must maintain their designated polarity(in our application).
The reasoning for why the live conductor is fused, and why the neutral should not be, is because the neutral needs to maintain a closed circuit for fault currents to trip the breaker in case of a fault. If the neutral was fused, the fuse interrupts this function.
The other reason the neutral is not fused is -- even though the circuit would be broken, there might still exist a live voltage hazard inside the equipment potentially putting someone at risk of shock. This makes a good case for RG's recommendation that the Live conductor should be fused right at the entrance to the device generally.

Do you happen to know if that's recommended or even mandated in any national / international standards, eg EN60335 and related docs.
Having retired, I don't have access to that stuff anymore.

I don't have a copy of the new UL adopted IEC standard 62368-1, so I'm not sure what the new requirements are. But, I know that Fender, and other manufacturers began wiring Line-Switch-Fuse at some point in the Modern era. The Hotrod series is wired this way for sure, however, I just did a quick check of some of those Fender models we have in the shop, and I couldn't find any UL listing on them. When I get home, I can check the older UL standard to see.
I'm curious if anyone has access to IEC 62368-1?

[R.G.]

Do you happen to know if that's recommended or even mandated in any national / international standards, eg EN60335 and related docs.

My last brush with actual standards was IEC60650. I think they've updated standards since then. I doubt that the standards actually specify "put a fuse first" or similar at all. They're really careful not to tell you what to do, only an end-result requirement like "after the failure of the worst-case component no electrical hazard shall remain" or some such. They never say "this is safe", only "if you get it wrong, the testing lab will fail you". It's one of my personal gripes with the safety standards biz.

In my designs, I always put fuse first because I thought that if the switch fails and puts live voltage on the chassis, the fuse would blow and protect the user. There's a case to be made for putting a switch first to prevent hazards to service personnel, though. My testing lab turn-downs were always for things other than fuse-first, so I stayed with it. I know that a fused IEC input connector is listed under many world wide standards, so I'm guessing that fuse first is a safe bet.

So, that’s what I’m going with. I already figured I’d do it this way, but I just wanted to see if anyone had another opinion, or if anyone knew why Vox wired Mains the way they did. The only answers I’ve found is that they did it this way because it doesn’t matter, or because it was more expedient to the layout.

I had to come up with my own rationale for why early two wire amps were wired in the ways they were. (Yes, do occasionally ponder things like this. ) Best I can come up with, the worst layouts are probably for wiring ease, as you guessed. For the early amps, they just ran the line cord in through a strain relief and grommet, then had to figure out where the wires went. With a switch on one side and a fuse connector on the other, it's easy to run one side of the AC line to the switch, the other to the fuse. Otherwise, they would have to run one wire much further, all the way to the terminal lug for the transformer primary. One on each side gives you a handy place to solder the line cords, and also a place to solder the transformer wires without adding a terminal strip or needing an extra lug. Shrug. It's just a theory.

[Phil_S]

I couldn't help myself from searching, though it was implied that you'd already tried. There is a paywall for the IEC standard. I'm surprised at this because it is not good public policy to restrict access to compliance standards that affect the general public. I'd put it in nearly the same category as public laws and statues. Can you imagine being arrested or fined for not complying with properly enacted public laws that you couldn't see or read? I don't understand the paywall. There's got to be some kind of bypass. You might be able to get a copy through your public library if you live in a big enough city or town.

[Phil_S]

I had to come up with my own rationale for why early two wire amps were wired in the ways they were... It's just a theory.

Remember, there was a time in the US when appliance cords were not polarized. In that era, there was no difference, so wiring could have been random. There may have been some carry forward in wiring practice when we changed to polarized plugs where it may not have mattered. A toaster comes to mind. I suspect "didn't matter" may have been casually applied for some time, maybe longer than we'd imagine and more widely than we'd think was appropriate.

[pdf64]

… I suspect "didn't matter" may have been casually applied for some time, maybe longer than we'd imagine and more widely than we'd think was appropriate.

In 1970, Fender were fitting earthed power cables but continuing the previous practice of fusing one side and switching the other. Thought at least the export version had the fuse on the live feed, after the DP switch.

https://el34world.com/charts/Schematics ... _schem.pdf

It’s seems they got their heads around it by late 71 (?). For the domestic version at least. The export model got the fuse and voltage selector on the neutral feed

https://el34world.com/charts/Schematics ... _schem.pdf

They finally got both right by 1977
https://el34world.com/charts/Schematics ... _schem.pdf

[R.G.]

I couldn't help myself from searching, though it was implied that you'd already tried. There is a paywall for the IEC standard. I'm surprised at this because it is not good public policy to restrict access to compliance standards that affect the general public. I'd put it in nearly the same category as public laws and statues. Can you imagine being arrested or fined for not complying with properly enacted public laws that you couldn't see or read? I don't understand the paywall. There's got to be some kind of bypass. You might be able to get a copy through your public library if you live in a big enough city or town.

Yep, that's a problem, all right. The safety biz has adopted many of the questionable features of commerce. In the USA, there are ... no ... laws requiring conformance to safety standards in general. The exceptions are OSHA rules about what you make your employees work with, and the continuous threat of being sued. There's no national law requiring safety certification. This is different in other countries, notably Canada, the EU, UK, Australia Korea, Japan, China and others where certification is not mandatory. The safety organizations have apparently always sold their standards, but the prices are pretty high these days. I think I paid something like $250-300 for my copy of 60650 back in 2006. That was at a discount distributor. IEC wanted about $400 IIRC.
I got mad about having to pay to find out what I was required to do, too. But with governments that support their effective monopoly on what's safe, the testing standards orgs can and do sell them, protected by copyright law.
I also got mad at the way that standards makers and testing labs will not tell you what to do to be safe. They will only inspect your work and say "that's not safe". The standards never say "do this and it is safe". That way if you (... or they...) missed something, you're on the legal hook for it, not them. The testing labs will only say that your work does or does not conform to their understanding of the most current standard.

There are even more gotchas in the safety game that I haven't mentioned.

[Interstellareye]

I’m reading all of these responses with great interest, but it seems like nearly everyone is commenting on best practice for fuse and switch placement. Although it’s interesting reading, respectfully, this is not what I was asking about.

The fuse is almost always on the Line side in these schematics (both sides switched actually). The fact that Vox use a DPST mains switch shows at least a deference for safety, doesn’t it? These are fairly over engineered amps, at least by the standards of the time. I don’t think it’s likely that they were careless with any part of the design.

What I’m still wondering, why do they always send the Line (fused and switched) to 0v and the Neutral to the PT Primary? Is it possibly because of the front panel voltage selector on the way to the Primary?

Is this such an innocuous thing to do, such a non-issue, that it’s not something anyone would even notice or question?

[R.G.]

The fuse is almost always on the Line side in these schematics (both sides switched actually). The fact that Vox use a DPST mains switch shows at least a deference for safety, doesn’t it? These are fairly over engineered amps, at least by the standards of the time. I don’t think it’s likely that they were careless with any part of the design.

What I’m still wondering, why do they always send the Line (fused and switched) to 0v and the Neutral to the PT Primary? Is it possibly because of the front panel voltage selector on the way to the Primary?

Is this such an innocuous thing to do, such a non-issue, that it’s not something anyone would even notice or question?

I'll give my opinion. I have over the years been exposed to many people on the web who were very sure that power supply components could cause subtle changes in tone that were not the result of ripple voltage or hum leakage problems. I have to tell you, I've tried but I can't come up with any way the incoming AC wiring can affect the amp sound except through ripple or AC hum. I would welcome a discussion of things I may have overlooked in coming to this.

If we accept that, then the exact wiring of the AC end of things needs to (a) power the primary (b) be safe and not kill the owner and (c) be the most convenient or most economical to manufacture. I firmly believe that variances in where the voltage selector is put once the safety and operational requirements are met do not matter particularly. As far as the power transformer knows, there is no difference between line and neutral. The transformer primary doesn't care. It only knows the incoming voltage and frequency. In the context of driving power transformer primaries, there is no zero volts and line volts - they're both just part of a floating AC power voltage.

I can't come up with any good rationale other than freedom to do it any way you like once the safety and performance items are covered, and a desire to make manufacturing speedy and economical, and possibly just go with intellectual inertia - we and they always did it that way, so it's traditional and let's do it again. It's not sloppy or careless if the exact details don't matter.